"VASSAL is a game engine for building and playing online adaptations of board games and card games. It allows users to play in real time over a live Internet connection (in addition to playing by email). It runs on all platforms, and is free for personal use."So, simply put it allows you to play tabletop games online, without the need for miniatures. You are required to download the correct module for the game you want to play, but they are all free and there is an extensive range of games available. For those in the Games Workshop hobby there is the 40K module, along with work being done on a Blood Bowl and Fantasy version. Or was. I have just read that the work on the modules is being closed (GW legal stuff), so it won't be available after 31st July 2009. I'd get the stuff downloaded if I was you!
I haven't personally played a vassal game, but I have watched one and it was quite well done. It involves a map, along with a .gif image of all the models in the game - you make your army and fight! It had its own inbuilt dice roller as well. In my opinion its a great idea. This allows you to either play with an army you normally wouldn't, or even playtest an army without having to buy the minis or proxy them in a game. Or even if you just want to have a game against a friend on the other side of the moon!
I would suggest gamers take advantage of this program, its worth the effort of downloading it.
Warmachine and Hordes
As I said above, there were plans for Blood Bowl, Fantasy, Epic but it appears that GW spotted this program and have done their usual thing of shutting it down. So, hurry!