BurnIn Test or similar bench-marking/system-test utility
Burn-in Test is a great little utility and worth paying for. I tried some of the freebies, but nothing gave me the 100% CPU loading I needed to test the effect of different CPU coolers, fans, fan speeds and case mods had on system temperatures.
In fact it may be possible to run this without installing anyway - I haven't tried since I started using PrtableApps, but it would be a great addition to the list.