Load this after you thoroughly read the release notes - no, seriously, take a little time and do that. And as always, test, test, test!

SP3 (Std, Dev, Ent)

SP3 Express

SP3 Express Advanced

SP3 express Toolkit

SQL 2005 Feature Pack Refresh December 2008

SQL 2005 BOL Refresh

SQL 2005 SP3 Read Me