Make sure you measure before you buy. My motherboard is a little higher than most and some of the bigger 120mm heatsinks don't fit. Measurements should be provided. I recently bought the following model and it works well on an i5-2500K:http://www.newegg.com/Product/Product.aspx?Item=N82E16835233081
The coolermaster recommended by wintermint is very good, though.
Frostytech makes a list of top heatsinks by noise and cooling if you need to bust out the big guys:http://www.frostytech.com/top5heatsinks.cfm