Exchange Server Version and Build Numbers

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