There are a whole bunch of these things..
Fantasy Grounds, OpenRPG, WebRPG, Ghost Orb, GRIP, Screenmonkey, kLoOge, and TRIS.
I'ts been a while since I checked, but there are probably even more of these virtual tabletop RPGs out now. Basically, they have evolved from IRC (Internet Relay Chat) roleplaying to a virtual environment where the GM and players can move their characters around on a virtual grid complete with virtual miniatures (very useful for visualizing combat!).
I've tried all that I've listed and although OpenRPG is not the prettiest graphically, it is the best one to handle other games that aren't 3.x/d20 (2ndEd, Vampire, Hero..etc).