Free, I don't think there's anything that compares to Thunderbird.. are you using the new version 3?
http://www.postbox-inc.com/
This one is pretty cheap and I've heard decent things about it, though I've never used it.
Having all my email online and available for searching from anywhere and having my read/not read status updated live from any device I'm reading from are two big reasons why I'm online only now.