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.