HGE, short for the Haaf's Game Engine, is a game engine popular with indie developers while being lax on the graphics side. HGE itself is a "a big pile on non-portable Windows source code", but Ryan cleaned it up and has made it publicly available. Originally he ported HGE to Mac OS X a while ago for a client, but when the need arose to port it to Linux, he started off with that Mac code and worked from there. Now the "hge-unix" engine, which is compatible with the Windows version, is publicly available.
Hammerfight, which is part of the Humble Indie Bundle #3, is one of the games that uses the HGE engine. HGE-unix is built with SDL, OpenGL and OpenAL. Details on the source release can be found at its new Icculus.org project oage.