Force: Leashed is a free first person gravity fiddler. To advance, you need to guide rockets to their targets using spherical potential fields. And no, it's not like Portal.
Force: Leashed was originally done for the 7DFPS challenge. After that, a more polished version was entered in Assembly 2012 GameDev competition, and it got the 2nd place!
Force: Leashed is built with the DarkPlaces engine, which is based on the original Quake source code.
It's not bad. A bit short, but the concept is interesting. It is unfortunate that you have to configure it manually via the config file, because it is completely capable of in-game configuration, but it's not hard to figure out.
"All packages include the GPL licensed game and engine source code."
Description needs to be updated; the second link seems to have a longer and better description.
Control config can be changed in:
~/.forceleashed/data/fl/config.cfg