This microsoft article details the various Exchange Server versions, including service packs.
http://support.microsoft.com/kb/158530
Some recent ones:
Microsoft Exchange Server 2003 6.5.6944 October 2003
Microsoft Exchange Server 2003 SP1 6.5.7226 May 2004
Microsoft Exchange Server 2003 SP2 6.5.7638 October 2005
Microsoft Exchange Server 2007 8.0.685.24 or 8.0.685.25 December 2006
Microsoft Exchange Server 2007 SP1 8.1.0240.006 November 2007