1998 saw the release of Alternity, a Science Fiction Roleplaying game from Wizards of the Coast under the TSR banner. While the cover says "Science fiction", it's interesting to note the inside cover page states "Rules for modern to far-future roleplaying games."
When it came out, it had the highest price point for an RPG to that date. At $29.99 each for the PHB and the Gamemaster's Guide, the books were also the most colorful. The production value is very high for the core books.
Alternity can be seen as a pre-cursor to d20 and even d20 Modern, (the latter which would include story elements, characters and settings from Alternity.)
The dice system for skill checks and such was a bit funky. You rolled a control die of d20, and then added or subtracted a situational die. The problem with the system was the goal was to always roll low. Traditionally, rolling a 20 on a d20 was a good thing and rolling a 1 was very bad. Here, rolling a 1 was desireable. The control die and situational die were rolled and added together, and compared to a target number.
Worse, because of this setup Alternity had the same problem some mechanics in early versions of D&D had was that a "bonus" was subtracted rather than added, and penalties were added. It's counter intuitive. If your boss said you earned a bonus this year, you wouldn't expect him to take money out of your paycheck.
The situational die could be adjusted by steps as well. You could improve a d4 to a d6 if the GM thinks you have a situational advantage (or bonus). Likewise, you may end up rolling with a step penalty, in which you'd look down the table a number of steps. Say the bad guy has good cover, the GM might rule you have a 3 step penalty to hit. Now instead of a -d4 bonus, you might end up with a +d6 penalty.
In addition, there's a degree of success. There are five degrees. Most of the time the positive three were noted. Ordinary, Good and Amazing. If you hit with an attack, depending on what you rolled, you could roll for more damage. A D&D short sword may be 1d6 for everyone, but in Alternity, if you roll well, you could end up doing more damage, and if you rolled an Amazing attack, still more, and may even change the catgeory of damage. (Short sword in Alternity, on an ordinary roll, d4 wound damage, on a good result d6 wound damage and on an amazing attack roll, d6+2. (I say here again, a plus in this instance is a good thing, but most of the time it's bad. Very confusing.)
There are three types of damage in Alternity, Stun, Wound and Mortal.
As you can see, it can get complicated rather quickly. But, let me throw in one more stat example to show how it flows.
A character has these as skills:
Modern Ranged Weapons 13/6/3
Pistol Rank 1 14/7/3
Now, if the character uses any sort of modern ranged weapon such as a shotgun, they would roll a d20 and use a base situational die of +d4. Using a pistol, however, only a d20 would be rolled as the specialty in pistols would have a base situational die of d0.
The first number before the slash is to get an ordinary result, the middle for a good result and the third for amazing.
Let's say this character uses a pistol. A roll of 15 or higher is a miss. 14 or lower is an ordinary hit, 7 or lower is good and if the roll is above a 3, it's an amazing result.
For a pistol, (.38 revolver) damage is listed as d4w/d4+1w/d4m. The letters indicate what type of damage (stun, wound, mortal) is done.
BLAM! A 5 is rolled in the first round, a good result, so d4+1 of wound damage is done.
BLAM! A 3 is rolled this turn (The dice think they're playing D&D and are rolling low on purpose!), good enough for an Amazing result, and d4 mortal damage is done this round.
That's the basics of the game mechanics. The PHB has plenty of other topics as well.
The first part has fast play rules to get into the action. Character creation follows and introduces a number of "new" races. Sure, one can play a human, but RPGs allow other races to be played.
Since it's a science fiction game, fantasy races such as dwarves and elves won't do. Aliens are introduced as playable characters.
The skill system is flexable, but unfortunately some key rules are burried in the paragraphs, rather than clearly laid out. There are broad skills, and specialty skills, which can be improved.
The PHB also covers everything from vehicles to computers, mutants to cybertech to psionics. Almost any sci-fi setting can be played. Indeed, many of the Alternity books are inspired by elements from popular sci-fi shows and books.