GameMode is a daemon/lib combo for Linux that allows games to request a set of optimizations be temporarily applied to the host OS and/or a game process. GameMode was designed primarily as a stop-gap solution to problems with the Intel and AMD CPU power save or on-demand governors but is now host to a range of optimization features and configurations.
Nvidia GPU Rom editor that when paired with a nvidia rom flashing utility, such as nvflash, allows you to modify the bios contents to allow OS-independent overclocking, underclocking, unlocking restricted features, etc.
AOT4L is a Qt4 based monitor and overclocking tool for ATI (TM) based video cards under LinuxOS which loads into system tray and stay runing all the time. It has support to read and change fan speed, memory and core clocks, thermal sensor values.