Thunderbird

If you use Outlook Express or Windows Live Mail, stop. Now. I mean it. Thunderbird is better in practically every way.

If it sounds like you just read this before, maybe it's because Thunderbird is made by the same team that developed Firefox. Microsoft has stopped development on Outlook Express, and it shows. Thunderbird has built-in spam filtering, an RSS newsreader, more advanced searching, message color-coding, and the ability to group emails by conversation. To help make the transition easy, Thunderbird can automatically import all your Outlook Express settings.

It's better than Outlook Express. Use it now. 'nuff said.

http://www.mozilla.org/products/thunderbird