Update: after I originally posted this article, my friend Greg over at CodeTempest showed me his favorite, which may my own new favorite as well. It looks better organized and more likely to be maintained.
Official Microsoft Reference Pages
Build Numbers for SQL Server 2008 R2 (Pre-SP1)
Build Numbers for SQL Server 2008 SP1
Build Numbers for SQL Server 2008 (Pre-SP1)
Build Numbers for SQL Server 2005 SP3
Build Numbers for SQL Server 2005 SP2
I don't track anything older than this, because frankly, you should really consider upgrading.