NvClock is a utility with which you can tweak your nvidia card. You can adjust the speeds of your card. Soon you will also be able to do other things then just overclocking. You will be able to tweak OpenGL settings and your xfree86 config file.
AOT4L is a Qt4 based monitor and overclocking tool for ATI (TM) based video cards under Linux OS 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.