I have/use Everest Ultimate (Lavalys) for stress testing, a little benchmarking, and for general monitoring. Way overkill for just voltage, but if you're running Vista it comes with a very nice,
(albeit only 'workman~like' in appearance), sidebar gadget you can configure to monitor whatever combination of temperatures and voltages you like.
Instead, for general usage I'd recommend CPU-Z:
http://www.cpuid.com/cpuz.php