Already mentioned by sucuri2 in another thread, but roundcube (http://roundcube.net/) looks much more attractive. Still not GMail, but much nicer than squirrelmail.
What are some of the plugins that you find useful?
I also have managesieve installed, and there's a plugin for it too, so with imap, inbound email gets automatically sorted into folders for me, and I can control that from RoundCube. The filtering is actually way more powerful than anything Gmail can do (last time I checked).
Those are the two "must-haves" IMO.