Street Fighter Online
I just came across this multiplayer, online version of Capcom's X-Men vs. Street Fighter arcade game and it looks very impressive. Available as a Shockwave version to play in a browser, or as a download in executable form, Street Fighter Online features a chat room interface for creating and joing games with up to 4 other street fighters. A single-player story mode is also available if you're not up to playing against real people. The game was started a little over 3 years ago, and has amassed quite a large following as evidenced by the game's active forum. Some say the reason for its popularity is because it's free. That, and because "people really tend to like street fighter." You be the judge. Click.