What is a good after market CPU cooler to replace stock?

Cooler Master Hyper 212+ - IMO the best budget cooler...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

While its not the best cooler I agree with gkay. The price vs performance for the 212+ is excellent, and perhaps the best thing about it is the fans are easily replaceable. So if the cooling with the stock one isn't good enough, just add a second, or replace with a faster or quieter model. Best of all the performance IS good. More impressive when you consider its small size.

An alternative is the scythe mugen which is one of the best performing HSF's out there. However, it is very wide so if you use ram with heat spreaders it will block first slots.

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

+1 @Gkay09. I just installed a Hyper212+ on a 740BE and it is very quiet, and seems to cool quite well (was around 30c before I unlocked the 4th core; unsure after that).
Otherwise, choose something with a 120mm fan; the 92mm fan on the AC Freezer whines like a monkey in the Wal-Mart candy aisle.
Check out www.frostytech.com for other HSF recommendations with reviews and audio clips.