I came across this site a couple months ago, and overall I like it: http://fabletop.com/ Simple, easy to use, great for quick pick-up games. Biggest problem is there is very little you can do to customize the system to suit your tastes. The system in place is good, not saying otherwise, but it makes playtesting my own system on the site rather difficult. Also, though there were a small number of people running games when I first joined, the site seems to be languishing right now, possibly due to a lack of GMs. If you already have a playerbase to bring to the site, not so much an issue, but it can be hard (as a player) to find a game when you're available to play.
So I also came across this site: http://roll20.net/ Not quite as simple and certainly not as quick and easy to set up a game, but the customization is much more universal. Dice are customizable, and I can actually use my own system on the site. Plus, the option of voice and video chat between players is a nice touch. I played around with it a lot tonight, and I'll probably end up using it to run games soon. It seems vastly more populated than Fabletop, though that's less of a concern for me there since I'll be playtesting my own system.
On a side note, I may need to tweak the bonuses for weapon skills. It looks as though I may have been a wee bit generous.
