FreeGish is an open-source, freely redistributable version of Gish, a physics-based platforming game featuring a ball of tar.
You can see a Youtube video of it here.
Gish was featured on the first Humble Indie Bundle. When HIB#1 made over a million US dollars, the sourcecode to Gish was released to the public. I was one of the people who took the code and attempted to create a freely redistributable game out of it. As I am no graphics artist, I used the excellent TRaK5 texture set for the textures. There are currently about five singleplayer levels for it, and a few multiplayer (on the same machine) levels. I mainly worked on textures and the singleplayer levels. Others have worked on level formats and various other things.
FreeGish uses a slightly different map format than was used in vanilla (retail) Gish. Vanilla Gish baked textures into the levels themselves, which was a pain when replacing textures. FreeGish's new format can reference textures by filename. To allow us to convert levels, I've written a conversion tool, and a tool that mapped file hashes to filenames which would allow us to easily convert all regular retail Gish content into content fully compatible with FreeGish.
FreeGish development is currently seemingly on hiatus, presumably due to lack of interest.
FreeGish is in the repositories of various Linux distrubutions. It's part of Debian, available in Arch Linux's AUR, and will be available in Ubuntu 11.11's repositories.
My own Github repository of FreeGish can be found here, a list of people directly involved in the FreeGish project can be found here.