GMail Loader is a cross-platform Python utility that can import mail messages from several mail storage formats (including mBox and MailDir) straight into your GMail account. It doesn’t currently support Outlook’s PST format, but there are utilities for converting PST to mBox format (see below). Eventually, support for direct access to IMAP will be added, which should allow you to transfer mail from just about any system into GMail.
I’ve got years of old mail archived away from different ISPs over the years. I think it’d be interesting to push all that stuff out to GMail and re-read the discussions I was having in 1996.