Flash Flash Revolution, or FFR, is an online rhythm game that tests a player's ability to keep up with a song's rhythm. It consists of a series of songs that have been pre-programmed with colorful arrows that correspond to the direction and timing of the beat. The player must hit the corresponding arrow keys on their computer keyboard when the arrows reach the bottom of the screen. Points are awarded for successful timing, and a score is given at the end of the song. The game is designed to be addicting and fun, with an ever-growing library of songs and difficulty levels. FFR also has a vibrant online community, with players competing and chatting with each other.
Discontinued Project Page archived at Google Code.
I think StepMania is a good alternative to FFR because it is the best version of FFR.