THANG Online
It's an MMO in development, but the developers don't speak particularly good English, so I can't tell if it's in open beta or actually free. They have a system where you can buy ingame equipment with real money, so it might actually be free... I'm not sure. It's pretty fun, if a little basic at the moment.
Flash Flash Revolution
A Flash version of Dance Dance Revolution. Extremely addictive, and free.