Depends on what you are overclocking.
CPU, RAM, and Northbridge: Overclocking is done in BIOS. Tools you need in the OS is a temp probe, something on the lines of Realtemp, a stability tester, prime 95 and Intel Burn Test does well at both.
GPU: There are several, but Trixx and the Gigabyte utilities are slightly better than the competition. For stability testing, I use Haven benchmark on a 30 pass loop. 3d mark 11 will also do a decent job of testing. You still need a temp probe, 100% of OC utilities will have one for this.