One thing that all VTTs need is a good map. Gone are the days when you were the only one that saw the map for an adventure. Now for every encounter you need a map that the players can see.
There are some great sites for maps. Here's a short list:
RPGMapshare - a huge repository of maps, each with clear details on the usage.
Dunjinni User Forums - lots of great stuff posted here by the Dunjinni mapping community. There's certainly overlap between this and RPG mapshare. One of the most useful features are the items. If you want an item for a map, be it a house or a howitzer, you'll probably be able to find a premade one on here somewhere.
Fantastic Maps - I maintain a gallery of my own maps online. The Free Stuff section of the website has some creative commons licensed stuff and the full galleries have a lot more that's fine as long as you only download it for your own home use. You can also find many of the maps at high res sized for use with a 100px grid in your VTT of choice on RPGNow for around $1.39.
The Wizards Map Galleries - the gallery of maps from past adventures and supplements, conveniently in labelled and unlabelled forms. Be warned that some of these will need careful resizing to work in your VTT of choice, so don't leave it to the last minute to drop them into your adventure.
Post any others you've got and I'll try to keep this post up to date.