The Open Saucy Haiku OS has added support for the newest AMD Radeon graphics cards. Haiku OS, which is BeOS compatible, can run for the "Mullins" and "Hawaii" AMD graphics processors using its basic Radeon_HD graphics driver.
So far it can only manage basic display support and is untested but it should work for a "a lot of stuff" due to common support with other recent Radeon GPUs.
Linux has Mullins support with Linux 3.15 while the Hawaii support has been in the kernel for a while. There's also Mullins Gallium3D support for these newest APUs.
Open-source Radeon R9 290 "Hawaii" OpenGL/3D support, remains broken with the AMD developers admitting it's not a high priority and AMD R9 290 users are best off with the AMD Catalyst Linux driver for now.