Gentoo Linux mainly, though I do have partitions with Debian and Ubuntu that I use to test software on. I just recently installed a new pure ~amd64 Gentoo using gcc-4.0.2 and modular xorg and despite expecting a myriad of problems happily ran into very few, the biggest of which is direct rendering still not being supported for my radeon X800 card which leaving me no choice but except using the closed source ati-drivers for decent 3D. Oh, and I can't run Civ on that partition either because wine and xorg aren't playing well together.