Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
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.
AMDOverdriveCtrl is this new open-source utility for OverDrive overclocking of Radeon hardware under Linux. It's not an official program from AMD but was written by a Linux user against AMD's Display Library API, which they released for Linux in 2009. The program was announced this morning in our forums.