prime95
its the most well known, and probably the most reliable.
i think its best because it doesn't hinder you from doin other stuff while its running, it sets priorities lower... so it only uses up all the cpu that's left after the processor is done doing other processes. works flawlessly.
http://files.extremeoverclocking.com/file.php?f=103
one time i was running dual prime95 (cuz i have a dual core proc) and playing unreal tournament 2004 (no lag) all at once
