RivaTuner is a very cool tweaking and overclocking program for Nvidia graphics cards. It has a lot of features and allows you to adjust the speed of your GPU along with fan control and DirectX and OpenGL tweaking.
I use it on my Nvidia card and can safely overclock over 100Mhz on my GPU, and the DirectX controls are very handy as well. You can create different profiles and save them to activate when windows starts. Again though, it only works for Nvidia cards, so sorry to you ATI people. Get it now.
My Computers
10 Most useful Programs for Windows
Rasterbator
