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This is intended as a guide to quick multiclass dips - not MCing to get a paragon path, and not MCing because it's core to your build or grants some nifty racial feat, but the sort of thing that gets overlooked frequently in the race to tune everything - things that you can get by taking a multiclass that are good for almost everyone, or at least for a great many people, and feats and powerswaps that can add a whole new dimension on to your character no matter who you are. It is not particularly intended as a comprehensive guide. It's intended as a place for people who don't know what to multiclass into to go for insight and ideas.
Note that, with the free skill training, multiclass feats are almost never a *bad* idea. Unless you have a build that is so feat-starved that it cannot afford even one, the question should not be "if", but "which". The strength of MC feats as compared to normal feats is one of the reasons people tend to like bards.
(Throwing in first thoughts on grading. Please feel free to weigh in on differing thoughts on value and category names. Also, please do offer corrections if I have things wrong, and any pertinent data you think might belong. Many of these are from memory, and some are from memory of books that I don't own. I'm sure that I'm missing some of the attack-power MCs, for example)

General Types:
- Take a healing MC. Having a one-per-day healing surge use + bonuses for a minor action can easily save you or a party member in a tough spot. Having enough characters so multiclassed can mostly make up for the lack of a party leader, if you happen to lack one. (bard/cleric/warlord/artificer) Special note goes to the cleric on this one, because they get two of these in a way. The cleric Channel Divinity MC (DP) can be taken for Healer's Mercy, which gives a whole *lot* of healing if you use it in the right circumstances - and if you are in "the right circumstances" in this case, you'll be happy you have it. It's costly to use, and 1/day, but if your party needs more desperation healing, this will definitely give it to you. It doesn't give a skill, and it doesn't give implement use, but.... Non-cleric divines can take a somewhat different feat that will add that to your channel divinity list, and let you pull it off once per encounter if you need to.  Special mention goes to the Artificer because *his* "surge use" doesn't actually cost a surge - making this MC pretty much equivalent to a number of level 2 leader utility powers.
- Take a striker MC. There are a number of MCs out there that will up your DPE pretty directly, usually by adding damage on to your existing attacks, usually for a round or two per encounter, usually for the cost of a minor action to set it up. Damage is always good, but these generally don't give all that much of it. (Avenger, Barbarian, Warlock, Ranger, Rogue, Sorceror). Special mention goes Disciple of Divine Wrath (Avenger, PHBII). If you happen to be a melee class, the avenger feat can give quite the significant single-target boost for a nova, once per encounter. Of course, the other Avenger feat is just plain better, but it's special enough to be mentioned separately.
- Take an attack-power MC. Gives you an at-will of the class you MCd into as an encounter power. Useful primarily as a utility effect - it generally won't be any more powerful than your own, and may suffer from MID/MAD issues, but it can let you do things that your own class doesn't do as well (or at all), and that can be helpful. (Wizard, Warlock, Sorceror, Invoker)
- Ritual Caster: Ritual caster as a base feat comes by itself. Ritual caster as a multiclass feat comes with a free skill. If you wanted to be a ritual caster in the first place, then having a free skill come with it is always nice. (wizard, bard, invoker)

Specific Feats of Note:
- Druid: - it's like an attack-power feat, except that it comes with the ability to transform into an animal. This can be useful in all sorts of noncombat situations for evading people who are chasing you (Bluff check [easy] to convince them that you are actually a dog.) and, if you don't use your minor actions much can let you cash in a minor action one round for the ability to use a minor to shift 1 in the next. Druid also comes with a feat or two that significantly increases stealth and movement in beast form - meaning that taking this as the party scout can up your ability to get in, get out, and not raise alarms significantly.
- Battle Awareness: (fighter, from MP) - if you've got the 13 wis and 13 str, you don't use your immediate action much, and you're a melee character with a decent melee basic, this is pretty much a free attack once per encounter.
- Hero of Faith (Avenger): - Hero of Faith (Avenger): - Hero of Faith (Avenger): Honestly, this one is good enough that it almost calls for a notch somehow above sky blue. Pick an enemy. Spend a minor action. Until the end of the encounter, if the enemy is adjacent to you and no one else is, you get to roll twice on every melee attack you make on them. Note that according to various bits of math theorycraft, rolling twice is equivalent to or better than a +4 to hit in almost all actually encountered cases. It also nearly doubles your crit range. The only limitation is that you have to have a 15 wisdom. This would be a strong multiclass feat as a *daily*. As an encounter power, it is overwhelmingly good.
- Shaman: Spending a standard action in the middle of the fight to summon your spirit is a bit much when you can't really attack with him, but you can summon him outside of combat and have him follow you in. Once he gets into combat, he's a highly mobile 1 square obstacle to enemies (but not allies) that takes exactly no resources to maintain, and grants light cover to allies (but not enemies). If the enemies don't attack him, he's a small but significant block of always-on battlefield control for a feat. If they do attack him, they'll probably waste a healing surge's worth of damage or more between damage underflow, damage overflow, and the 5 point reduction at the end. This becomes even more entertaining if multiple party members do it, and you can start arranging your little friends in walls. Plus, once per day you get your wis bonus, untyped, to a skill check (if you qualified for the feat in the first place, it's at least +1), and you still get the extra skill.
- Paladin: a marking feat that really is a cut or few above the rest. Unlike every other marking MC in the game, you can get this one to stick around from turn to turn - which means that you can use it as a psuedo-defender to suddenly get a lot more defendery, or as a sufficiently mobile ranged striker to mock and punish some poor lout for his inability to hit you.

Nifty Multi-feat Combos:
- Sorceror flight: a sorceror multiclass feat, a level 16 utility powerswap, and a good dex can get you a fly-and-never-fall speed that will let you flit around the tops of tall rooms, firing bolts of flamey death (or icy death, or arrows or crossbow bolts or whatever) down upon anyone you happen to dislike. Also, being able to fly as far as you like just because is really quite handy in a number of noncombat and combat-prep situations. It loses some of the combat applicability in cases where the ceiling is too short, and you're losing any cover you might have had against their ranged casters, but it can still be quite potent.
- Sorceror AC: It comes a little late, but it's a beaut. Any str-based character who MCs to sorceror and makes it to level 22 can powerswap for platinum scales. Pick an encounter. As soon as an enemy takes a swing at you, every single one of your defenses goes up by your strength mod as an immediate interrupt - which, by that point, should be about +7 - for the rest of the encounter. For two feats and a utility power, that's not bad. It does take your immediate action for the round, which nonpaladin defenders might care about - but in most cases, if the object of your affections is hitting you already, you won't need that immediate action until your next turn anyway.
- Warlock Teleport: a warlock multiclass, a lvl 10 utility powerswap, and enough teleport-boosting gear can get you a solid at-will teleport as a move action. This can be excellent for deep-striking melee characters into the enemy back ranks - or for escaping from melee characters if you happen to prefer the back ranks yourself. Note that getting bonuses to teleport is a lot easier than increasing both your move and your native shift distance. Teleports can *also* be quite handy on skill challenges and the like. Note that dwarves in particular, with a moderate gear investment, can get at-will teleport speeds that equal or exceed their move speeds (and heavy-armor gnomes can do so almost trivially... which raises the question of why you would be playing a gnome in heavy armor in the first place, but anyway....). It gets pushed down a bit because of the gear requirements. For anyone who has the equipment slots to spare and no real plan for them, it becomes sky blue
- Leader Healing Utility (warlord, cleric): if "Take a healing MC", above is useful, but once per day just isn't enough (entirely possible if you have no real leader - especially if you have no real leader and only a few people are willing to MC into leader classes) you can significantly upgrade your ability to pretend to be a leader by swapping for one of the surge-use encounter utility powers from your MC class.  -- Special mention on the warlord side goes to Rousing Words for being a level 6 power that does this for up to two surges at once. It doesn't carry any bonus health, but if you've gotten to this point you care more about surge use than surge efficiency anyway.  -- Special mention on the cleric side goes to Word of Vigor at level 10 - letting you grant a surge use to yourself and everyone next to you, and giving an extra 2d6 bonus healing on top of that.  -- In a similar theme, if you don't want to make the powerswap (or, if you already have, and still crave more), elves/halflings who've MCd cleric can take the Blessing feat for Corellon/Avandra respectively, and hand out a healing surge use once per encounter for free when they use their racial power.
- Paladin Weapon Blessing: This one is a little heavier on the requirements, and a little heavier on the cheese than I normally use here, but when properly supported, Bless Weapon, the level 2 paladin utility, can be amazing for just about any melee character. If you can find a way to consistently inflict radiant vulnerability, it's an extra +1 to hit and an 18-20 crit range off the weapon of your choice, along with the extra vulnerability damage (and if you're getting here through the paladin marking feat that radiant vulnerability will make you extra sticky too.) The easiest way to get that vulnerability is to start out as an avenger or melee priest and take the Solar Enemy channel divinity power. (total cost: 1 MC feat, 1 powerswap feat, 1 lvl 2 utility, 1 channel divinity feat). For nondivine melee characters who are interested, you can get the same effect once per day if you take the channel divinity multiclass feat. You can also go with the Morninglord PP or (if half-elf) a divine at-will that inflicts radiant vulnerability through Power of the Sun. Of course, any of these can also be inflicted by other members of the party as well, if you can get their help on it.

the_LICH wrote:

It is level 16 but it does not exactly give you a fly speed. It just allows your target (you or an ally) to fly a number of squares equal to your dexterity for a move action. Which means that if you stop and cast or end your round at midair, you fall.

uhhm, as long as you have a dex mod of +2 or higher you qualify for the flying rules, which state that you need to fly at least 2 quares in your turn or crash, so yes, it technically does give you a fly speed, sinc eyou can target yourself with it every turn as a move action to fly dex mod squares. Casting does not cause you to crash though, nor does stopping in midair, since it doesn't state anywhere that you fall if you take those actions.
-flying rules, page 47 of the Dungeon Masters guide