OpenITG

Open source clone of In The Groove 2, the wildly popular rhythm game used in arcades.

In fact, it even does everything In The Groove does and can be installed on ITG cabinets.

Features
OpenITG was based off of StepMania 3.95 up until beta 4 when it moved to the SM4 codebase.
 * Save scores and other info to a USB stick
 * Load songs from a USB stick, playable on the machine
 * Marvelous timing
 * Basic debug menu and debugger features not present in SM3.9

It remains popular to this day and is still in use, but is outdated and succeeded by NotITG.