It’s not often that I get excited over an email client. Honestly!
However, since discovering RoundCube (or rediscovering as it were, because I had known about the project for about 6 months prior to becoming a convert), I just love how everything comes together on this open-source project still in 0.1-beta phase. RoundCube is an IMAP webmail client which has an AJAX-based user interface that is both user-friendly and responsive – not unlike GMail – and no redundant features, I may add. A quick installation guide of RoundCube on your server can be found here; here is another guide on installation for Site5 servers.
Finally, a good list of Web 2.0 softwares.