The tools and libraries he recommended for game development are all open-source and available for Linux -- most of them are cross-platform as well. Below is the list of Ryan's recommendations along with a few notes from his talk that I attended.
SDL - Enough said... SDL 2.0 is finally on the horizon with many improvements over SDL 1.2. SDL 2.0 details will be saved for another Phoronix article.
OpenAL - The OpenGL of audio.
PhysicsFS - One of Ryan's many open-source projects, a library for virtual file abstraction/access. He also explained the name of PhysicsFS (as it has nothing to do with physics), but it came when he was writing a game engine but he pulled it out as it was the only usable thing from the unreleased engine.
Open Dynamics Engine
Ogg Vorbis - No licensing burden like MP3. Ryan hates software patents (obviously).
Ogg Theora / TheoraPlay
MojoShader - Another Icculus project.
Valgrind - "Improve your life tonight!"
Clang - "State of the art in compilers... Static analyzer... Compiles two or three times as fast." Ryan really enjoys Clang for its static analysis abilities and other features not supported by GCC. Even though the binaries they end up shipping are still built with GCC, he and other game developers have begun taking advantage of LLVM/Clang internally.
Blender - "I dont know anything about Blender." Ryan doesn't have experience with Blender but just had to recommended it as an open-source alternative to Maya.
Git - "It's like coming to Jesus... It's so much better."
Game Engine Recommendations:
Ioquake3 / iodoom3
That's the list of his recommendations from the hour-long talk. His 2012 Flourish slides are available from Icculus.org (PDF). There was a video recording of his talk by the Flourish staff, but it's not yet been uploaded to YouTube.
When talking to Ryan after his Flourish talk was when he commented on the remarkable improvements of open-source GPU drivers. That's also when he said he's talked with Valve but not at all interested in taking up a job with him to work on their Linux client ports as he doesn't want to relocate to Washington.