An open-source email program, sister program to Firefox. I started using this early this year because my Outlook Express started screwing up. Now that I've started using it, I can't get myself to go back to using OE!

Handle multiple email accounts, a Junk Mail filter (pretty cool -- you have to "train it" a bit, but works effectively), smileys, RSS reader (for you people with blogs)...

Give it a try! :)