Flash Flash Revolution!

so theres this pretty cool game (linked above) that is pretty much like ddr (dance dance revolution, or stepmania)
its single player
as well as multi-player
theres no download needed, its web-based.
theres no registration required, though for you scores, etc to save you need to sign up.

you can win songs dependent on your skill,
find songs through provided riddles called "token" songs,
stack credits to purchase new songs,
you have a multi-player (MP) rank,
plus alot more to the site with addition beta games, etc!