My friend Scott MacGregor is a cofounder at Postbox Inc -- he previously was the lead developer on Mozilla Thunderbird. They recently left Mozilla to work full time on Postbox, which is built on top of the Thunderbird core. Desktop Email Revitalized
Remember all those things that Gmail does well but other mail clients are kind of crappy at? Conversations in threaded view? Fast search? Extracting out links and other useful stuff like addresses and phone numbers? These guys got it right. Bugs are fixed, and fast
Especially on Mac OS X, which always had nagging bugs in the editor that never were fixed previously. I can finally paste screenshots into my mail editor window. Admittedly, Mail.app has always allowed this, but when you have multiple IMAP accounts with thousands of messages Mail.app is kind of a non-starter. Their public beta is downloadable as of today -- works great on OS X, and I believe they have Windows versions too. http://postbox-inc.com