Frets on Fire

Screenshots:

Background:
Frets on Fire is a free, open-source rhythm game that plays like Guitar Hero or Rock Band. The game comes bundled with a few royalty-free songs, but you can download more or make your own fret patterns and share them with others. The game also has many fansites each with its own community and songs to share. Several independent artists have created and distributed their music specifically for the game.

Gameplay:
The object of the game is to complete songs by playing the correct "notes" at the right time. Aside from this there is a feature called HOPO (Hop-on pull-off) which allows you to just press the fret keys for certain notes without the need to strum those particular notes.