HGE's Particle Editor
The proprietary games sale Humble Indie Bundle has a tradition of including open source engine releases.
In all of the other humble bundles there has always been at least one game that opened its source (in #2 it was revenge of the titans. in #1 it was penumbra, lugaru, aquaria and gish).- Fer C.
Bundle #3 is soon over and the *nix port of the Windows-only zlib-licensed engine HGE has been now released as open source as well. It is the engine used by Hammerfight from the Bundle. Visit the pages: hge, hge-unix.
The non-free BASS audio library was replaced with OpenAL.
The hge-unix readme states "10 I don't know anything about the Windows version of HGE.", which might mean that developers have to juggle two engine versions and audio libraries if they want to support the three main platforms. Or hge-unix needs to be ported back to windows systems.
You can read more about the port in the blog post HGE comes to Mac and Linux by Ryan Gordon aka icculus. Or perhaps watch his talk about Linux/open source gaming (again?).