Prime
Memtest
Sisoft Sandra Burn In utility
Fish or Seastorm screensaver
Downloading a large .pdf file also seems to stress out CPU's
Some people suggest running these utilities for hours and hours at a time but that's not always a good test of system stability. Go ahead and run these utilities but a few passes of usually enough then switch to a different utility.  Once these utilities seem happy the next step is to just put your system to varied use. 
Having a system take whatever you "throw at it" is the only reliable stress tester.