As many of you know, I love Firefox. I use it for about 95% of my web browsing (that other 5% is at work where they can't manage to write apps that work on anything but IE).
Anyway, I recently got Mozilla's Thunderbird for an email client at home. I have Outlook, and have the 3 email accounts I check and my wifes on it. However, every time I went to send an email, I'd forget to change from the default account (my wifes) to mine, so it always looked like she was emailing people.
So, to reduce confusion (mainly for the people that never understood why my wife was emailing them), I downloaded Thunderbird. And so far I like it.
While looking at add-on's and the like tonight, I saw reference to 2 other projects. The SeaMonkey Project and Calendar Project.
The SeaMonkey Project is "Web-browser, advanced e-mail and newsgroup client, IRC chat client, and HTML editing made simple -- all your Internet needs in one application."
The Calendar Project is broken into two, Sunbird and Lightning
Sunbird is a stand alone calendar program while Lightning embeds the calendar into Thunderbird.
I installed Lightning tonight, and linked it to my Google Calendars with a add-on I found called Provider.
It appears that all will run on Windows, Linux, and Mac.
So, don't have (or want) Microsoft Outlook, you might want to give one of these a try.