I've talked before about using an outliner as a PIM. Since getting my PowerBook I've tried out a number of OS X outliners with a fair degree of success.
Lately, however, I've wondered if I could pretty much achieve what I want just using the filesystem. I can set up a directory structure that mirrors the outline. I can use symlinks for items that belong in multiple categories. It's dead easy to put notes, bookmarks and arbitrary files under any item. Sharing of parts of my PIM info just becomes a file serving issue and I can version with a revision control system.
It should be relatively easy to integrate email as long as I can treat individual mails as files I can link to.
I can even set up a 43-folder tickler system and have a cron job that moves (or symlinks) files daily and monthly into an INBOX directory.