LinX/IntelBurnTest for maximum stress/heat on your cpu, keep an eye on your temps as it will run hot (Freeware)
Prime95 Blend Test as suggested above to stress your CPU/Memory/Northbridge/MCH. Not as stressful on the CPU as LinX/IBT but stresses different parts of your pc quite well. (Freeware)
If your overclock is stable after running Prime95 for around 8 hours and LinX for around 20 passes using maximum memory/stress level then you can consider your oc stable (run these programmes seperately, do not run at the same time)
SuperPi 1.5 for testing speed/stability of your memory (Freeware)
Everest Ultimate benchmarks are also quite handy to run to ascertain which settings can provide the most bandwidth/speed/ lowest latency. (Not freeware)
FAH (Folding at Home) is also very stressful, one of the Prime95 stress tests was a FAH gromacs core in older versions
Keep a close eye on temps and you should be OK.