RoundCube... the name may be an oxymoron... but this new webmail application ain't.

I just got through installing it here on my server and it seems to work well. The nice thing about it is the support for the
PosgreSQL database engine, which is more powerful and reliable than MySQL.
It has a nice interface, closer to GMail and the newer hosted webmail applications, using AJAX and all that web-2.0-ish goodness; less page-to-page navigation & reloading than the venerable king of open source webmail, SquirrelMail.
Plus, it uses IMAP internally, so you can connect it to any IMAP or IMAP-over-SSL host.