http://www.jamlegend.com/It's a really fun mock Guitar Hero flash game. The engine actually runs flawlessly, which is especially amazing since it's free.
Here's some gameplay of it.
http://www.youtube.com/watch?v=T7R6A4Rj1xcYou play with 1-5 as the frets, and Enter as strum. And if you have a wired Guitar Hero controller for Xbox 360 or PC, you can use that instead. (with Xpadder.)
You don't need an account, but it's more fun with one because you can level up and add friends. It even has live multiplayer, which you don't need an account for either.
This game can get really freaking hard sometimes, but there's hundreds and hundreds of songs from all kinds of genres to play, so there's always something you'll like. It's also a really great place to find some awesome underground bands. It's worth checking out.
Here's my profile.
http://www.jamlegend.com/user/J4L.S.DK_JLAdd me if you do make an account.
EDIT: 5,000th Games topic. =)