AMD has allowed their Radeon GPUs to be overclocked on Linux since 2008 when using their Catalyst driver with OverDrive support. Previous to that there was Rovclock for overclocking select ATI Radeon ASICs using an open-source program along with support for tuning the video memory timings and other options, which was a program written via reverse engineering. The Catalyst Linux driver supports OverDrive manipulation of the core and memory clocks, which is enough for most enthusiasts, but if you've been looking for more extensive features there is a new option.