SpeedFan is a freeware program that monitors voltages, fan speeds and temperatures in your computer. It is a great tool for overclocker like me. You can monitor your CPU cores temperature and CPU fan speed too. Its main features are monitor fan speed. However, it does not end here. It has other useful features like access S.M.A.R.T. info on hard disk, change the FSB on some hardware and control fan speed. It can be used on Windows 9x, ME, NT, 2000, 2003, XP and Windows Vista. It works with Windows 64 bit too.
As you see, above picture shows Intel Core2 Extreme quad-core processor QX9650 running in 4 cores. The are CPU usage percentage based on each core. Same apply to the cores temperature. Core1 is burning at 51C. It also show the CPU fan speed at Fan1. You can adjust the fan speed. However, I am not sure why that I cannot do so with my Cooler Master Geminii S (風神鍛) CPU Cooler. Further study is required. Too busy playing games lately.
In short, I use SpeedFan to monitor CPU cores temperature. It helps me to determine max CPU speed with suitable temperature that I can overclock.