Is there an open source alternative to Microsoft Outlook?

Is it possible to be free of Microsoft Office if you are an Outlook user?

A question from the Hardware 2.0 mailbox that I thought I'd share with you for comment:

Question: I used to be a big fan of Microsoft Office, however, for the past few years the only thing keeping me hooked on the Office suite is the fact that my entire life is contained inside a couple of Outlook .PST files. Not just my email, but my appointment, tasks, calendar items and contacts.

Do you know of an open source alternative to Outlook that can take in .PST files and allow me to carry on my day. Reentering all my data into a new applications just isn't feasible.

I've looked before and come up empty handed. For email there's Mozilla's Thunderbird, but that only handles email.

Any ideas?