You will not find any software listed that allows you to overclock your processor from within Windows.  Although some of the "soft" overclocking tools are useful for seeing how far you can push your CPU before commiting to it in the BIOS if you have a fairly modern motherboard with a good overclocking BIOS chances are that there is a fail safe option that can be enabled to prevent any permanent damage to your system.  Because of this and because some of these tools can cause damage to your OS they will not be listed.  If you know of any other good utility please feel free to list it. 

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*Graphics card overclocking tools:  Please list these in a seperate topic to avoid confusing those folks who are new to overclocking.  Thank you.

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Motherboard Monitor:  Displays temp,voltage,frequency info. on your desktop. Free

http://www.majorgeeks.com/download.php?det=311

CPU-Z:  The standard overclocking validation tool provides detailed information about you processor,chipset,memory,motherboard,voltages etc.  Used to "validate" your overclock. ~ Free

http://www.cpuid.com/cpuz.php

Prime95:  One the essential stress test programs for testing the stability of your overclock if your not stable this tool will let you know in a hurry.  Many overclocking data bases,sites/forums require you test stable with Prime95 to be considered a successful overclock. ~ Free

http://www.mersenne.org/freesoft.htm

SuperPI:  Another essential overclocking utility used to test speed and stability.~ Free

http://www.overclock.net/downloads/138140-super-pi-mod-1-5-a.html

Memtest86:  A must have to test your memory for stability and errors. ~ Free

http://www.memtest.org/

3DMARK:  A very poular benchmarking utility available in both a free (limited) version and a full featured paid version, another standard of the overclocking community.

Sisoft Sandra:  A standard of many overclockers and overclocking sites/communities provides detailed system information,a wide array of benchmarking tools, a burn in utility and much more.  The lite version is free (no nags,pop-ups,adware etc.) and has many the features included in the paid versions.

http://www.sisoftware.co.uk/

 PC Wizard 2007:  A very lightweight and easy to use benchmarking and system information program. Good for a beginners intro. to benchmarking. When minmized to the system tray displays detailed information on your desktop about core temp,CPU temp,core loads,Vcore and frequency, i use this program mainly for this function alone as i prefer it over motherboard monitor. ~ Free

http://www.cpuid.com/pcwizard.php

Overclocking of any kind can lead to hard and software failure or damage use caution and proceed at your risk.

AmericanNightmare