The Frequency series is a trio of rhythm games made by Harmonix.
Basically there are 5 - 7 note tracks and they are shuffled between (with players controlling a space ship) and hit the notes in the track. Once a section of notes is cleared, players move on to the next track (each track represents an instrument).
Frequency and Amplitude (1 and 2 of the series) has a feature that lets players remix all of the songs in the game, these could then uploaded online and played against others.
Harmonix has gone on to develop several games that resemble the gameplay found in Frequency and Amplitude. Phase, an iPod exclusive, features many similarities to both games. Additionally, Harmonix went on to develop both Guitar Hero and Rock Band, both of which share the same core principles, albeit using plastic instruments instead of a traditional controller.
Rock Band Blitz is a digital only title that was released by Harmonix in 2012 that played similarly to Frequency and Amplitude rather than Rock Band. Players used a controller and rotated between note highways that represented different instruments.
Both Audiosurf and Boom Boom Rocket feature gameplay elements that borrow heavily from the franchise, as well as other rhythm games before it.