Calling All Geek Types! Alternatives to Outlook?

OK, here’s the deal…I installed Firefox several months ago and I immediately ditched Internet Explorer. I also installed Thunderbird to try for my e-mail. I’m not quite as sold on Thunderbird yet, mainly because, while it imported my e-mails and addresses just fine, it won’t automatically import my rules and filters from Outlook. Also, I use Outlook (the big kahuna, not Outlook Express) for my calendar and other kinds of administrative things.

Is there a good alternative to Outlook for the calendar and other functions like that?

Your help will be greatly appreciated!