Flash games are interesting. Most are free; many are developed in people's free time. In my casual browsing, I've found so many creative ideas.
For example, I ran into this section on the ArmorGames website:
http://www.newgrounds.com/collection/rockout.html
It's their Rock Out contest (which ran in 2007), where they challenged people to come up with creative music/rhythm flash games. All the games are available for free to play. People have come up with some interesting cross genre concepts, such as "Bom Beat Battle"-- a fighting game.
I haven't been through all of them yet. I will say that "SuperCrazy Guitar Maniac Delue 3" (SC Guitar Maniac Dx 3) by Shinki was impressive in terms of how closely the gameplay reminded me of Guitar Hero. The notetracking was better than I've found in other flash-based rhythm games, too.
Although it's not a rhythm game, I have to mention a timing game I found on the sanriotown website (http://www.sanriotown.com) called "The Rolling Corocorokuririn".It's free to play but you have to sign up to be a sanriotown member. Yes, boys.. that means you get your very own sanrio email addy.
Gophers(?) on rolling tomatoes, buttons, and yarn may not be your theme of choice but the game is a lot of fun.