From Hilbert Hagedoorn:
RivaTuner is tweaking utility for NVIDIA and ATI display adapters running under Windows. The purpose of this utility is to give you access to all the undocumented features of Detonator and Catalyst drivers. Drivers have a lot of undocumented registry entries. In general, they cannot give you big performance boost but they can improve image quality and solve some compatibility problems. RivaTuner will help you to change all undocumented registry entries. It supports all versions of Detonators (now called ForceWare) drivers under Windows.