Overclocking is fun and beneficial, but can also cause major problems with your PC. A computer that is improperly overclocked can cause crashes, reboots, power-downs, data corruption, and eventually even permanant hardware damage. Again, it is important to diagnose these failures early on so that they don't escalate to bigger problems. This is why ANY good overclocker will have a toolbelt full of stress/stability tests to check their system with. An overclocked PC does you no good if it's not stable. Running games faster does you no good if they crash at random. Overclocking needs to be kept in check with stability testing whenever settings are changed. Different people will have different definitions of "true stability", but all overclockers should know how to test for it. Here is a list of commonly used stress test programs:
This is a distributed computing project, but can also be used strictly for stability testing. As with most programs, it uses strenuous math calculations to see if your processor ever produces failures (wrong answers). If there were a single universal "final stability test", Prime 95 could arguably take that title. It is widely accepted and very reliable.
This project is no longer being continued, but the tool is still incredibly useful. OCCT has two modes: A standard stability test and a torture test. The standard test runs for 30min and is a very good stability indicator. After the 30min test completes, it shows graphs of temperatures and voltages which is a feature that is not really found in other programs, and is very useful. The torture test simply runs until stopped and reports any instability, much like Prime 95.
SuperPI is actually a benchmark program, and gets its name from the calculations of Pi digits that it performs. SuperPI speeds are a good indicator of a processor's speed, but the tests will also report instabilities as they occur. This program can run very quick stability tests that are surprisingly accurate given their speed. This is another "must have" in my opinion.
This test is used to test just your memory, as the name implies. It can either be booted from a floppy disk or a CD-ROM. Once it is booted and running, it runs multiple test patterns on your memory until it is stopped, and reports errors as they happen. This test is very good for testing the memory alone, and helps to confirm that your memory will run at the speed and voltage you have set.