Here is a list of some Good Heatsinks
Thermalright Ultra 120 Extreme - Probably the best Heatsink Available
Thermalright Ultra-120 eXtreme CPU Heatsink - (Socket 775 / AM2) - FrozenCPU.com
Thermalright Ultima-90 - A smaller version of the Ultra 120, not as big but a good choice for smaller cases
Thermalright Ultima-90 CPU Cooler (Socket 775 / AM2) - Core 2 Quad & Extreme Ready - FrozenCPU.com
Tuniq Tower - A Massive Heatsink, it was considered the Best by many until the Ultra 120 Extreme hit the market
Tuniq Tower 120 Ultra Silent Universal CPU Cooler - Rev. 2 (T-120) - FrozenCPU.com
Noctua NH-U12P - A Favorite for a lot of Overclockers
Noctua NH-U12P CPU Heat Sink (Socket 775, AM2, 754*,939*, 940* F*, 771*, 604*, ) - FrozenCPU.com
Noctua NH-U9B - A smaller version of the NH-U12P
Noctua NH-U9B CPU Heat Sink (Socket 754, 775, 939, 940, AM2) - FrozenCPU.com
Xigmatek HDT-S1283 - Another Popular Overclocking Choice
Xigmatek HDT-S1283 Heat-pipe Direct Touch Universal CPU Cooler (Socket 754 / 775 / 939 / 940 / AM2) - $10 Rebate Until 7/31 - FrozenCPU.com
Enzotech Extreme-X 120 - Another Popular Choice
Enzotech Extreme-X 120mm Fan Quad Heatpipe Universal CPU Cooler (Intel & AMD) - FrozenCPU.com
And there are others but really it's a matter of available case space and your Budget requirements