An option that doesn't require you to pull the board is the Arctic Cooling "Alpine 64" or "Alpine 64 Pro". The two are almost identical. The "pro" just has a white fan and added support for daisy chained PWM control.
Its not a high end cooler, but its better than stock and its under $20 out the door. Its also quiet, even at full speed. It is harder to put on than the stock unit though.
Here is a link to a hardware secrets review of the unit dated January of this year and a couple of retailers currently offering it.
http://www.hardwaresecrets.com/article/894
http://www.ewiz.com/detail.php?p=FAN-ACALP6&c=fr&pid=b5602a1e0eae98bf3db3719d6879efd1410a77509560a93e1efa5a86ee08ea7b
http://www.directron.com/coolacalp64pro.html?gsear=1