Libpst

libpst is an open source program that can decode the email messages stored in a Microsoft Outlook "pst" file.

There appear to be two versions of libpst available.

The best appears to be at:

http://www.five-ten-sg.com/libpst/

This fork is only a couple months older, but won't read Outlook 2003/2007 format:

http://alioth.debian.org/projects/libpst/