Leadwerks Software raised $20k on Kickstarter in order to bring their 3D game engine to Linux. Leadwerks isn't as great as the Source or Unigine engines, but it's been around for a number of years, leverages Newton Game Dynamics, supports OpenAL, and presently uses OpenGL 2.1 for graphics.
Leadwerks Software licenses their engine on a $200 per user amount or a larger amount for a full source license to the 3D game engine. This Leadwerks Linux effort is focused around Ubuntu 12.04 LTS support, will feature Steam integration support, and there's also a stretch goal still going on at Kickstarter for OUYA Android game console support.
More details on Leadwerks Linux game development can be found via the Leadwerks press release after crossing their $20k threshold on Kickstarter.