Unity Technologies has released a free public beta of their Unity 4.0 game engine. Unity 4.0 has Linux publishing support among many other new features.
Earlier, developers had to pre-order Unity 4.0 to test out Linux support. With a free beta, interested developers can now test how well their game runs on Linux before going for a full Unity 4.0 license.
Do note that the Unity game editor supports Windows and Mac only, so the the beta is available for these platforms only. Using Unity's built-in Linux export support, developers can compile their game for Linux and then test it out on any Linux distribution.