http://www.mozilla.org/projects/seamonkey/

In case you didn't know, Seamonkey is the continuation of the old Mozilla suite. It consists of browser, e-mail client, HTML composer and IRC client - all in one application.

Those who are still using the Mozilla suite should definitely switch over as Seamonkey is built on the latest codebase, of course. Anyone who's using both Firefox and Thunderbird should at least consider, because not running huge chunks of the XUL and Gecko engines twice saves lots of precious resources. If you're just using Firefox along with another mail client (or the other way round), Seamonkey probably isn't for you.

I've installed it on my laptop now, because I was not satisfied with the mail client I've been using so far there. Of course, it's only been half a day, but I found no serious flaws, so the test run's definitely extended