The software that came with your motherboard, The bios (best one) or speedfan (I don't trust this much, as it can be very badly off (e.g. the processor in the 3 figure degrees C.)
everest is nice, good for all pc specs n all.
Your mobo displays temps also, as he said, but its hard to get temps while ur pc is in load, as it cools down quik, so by time ur in bios, cpu is cool already.