Prime95 is the freeware used by GIMPS, a distributed computing project dedicated to finding new Mersenne prime numbers.

FurMark is a very intensive OpenGL benchmark that uses fur rendering algorithms to measure the performance of the graphics card.

UNIGINE Benchmarks can be effectively used to determine the stability of PC hardware (CPU, GPU, power supply, cooling system) under extremely stressful tests.

OCCT is a stability checking tool, free for personal use.