If for some reason you have a need to know the current temperature of your CPU, or want a detailed look at many aspects of your hardware, CPUCool is worth a try. It constantly monitors the temperature of your CPU as well as your hard disk and reports on the results.
In addition to monitoring your CPU, it tells you a great deal of other information about the state of your hardware, including disk write speed, disk read speed, the percentage of your CPU used, and more.
It reports all this on a simple-to-read screen. It also displays the information in the system tray when minimized. The program says that it runs as a gadget in the Vista sidebar, although I never managed to get it to work.
Most people will most likely not find a use for this program--do you really care what the current temperature is of your CPU? However, if you overclock your CPU, you may well find it useful, because overclocked CPUs are prone to overheating.