depends on price you are willing to pay for. A generic example would be Scythe Gentle Typhoons, Gelids or Noctua f12, but they are costly. Slightly cheaper alternatives would be Swiftech Helix-120 PWM, Cougar Vortexs just as purely generic examples.
I own an Antec Kuhler 620 which I picked due to clearance issues with heatsinks, and swapped out the stock fan for two low rpm fans I had lying around. The results are not impressive, it's not dramatically better than the stock cooler at idle but the delta is quite a bit better; not to mention the fact it is nearly silent. It's not really a performance solution but it serves its purpose well.