OpenGW
Open Source recreation of the popular game Geometry Wars Retro Evolved
... as a basis for a custom-built arcade cabinet. Something that was true to GW:RE but added the multiplayer aspect of GW2.
Inspired by the Wii version of Geometry Wars I experimented with support for some pretty crazy custom-shaped levels with holes in them etc, but ultimately decided to stick true to GW:RE. See the Wiki for more information on that version and for additional notes.