Afterlogic Webmail Pro
It's a commercial product but they provide a full version with a 5-person license for free and the source code is included. In my case the only user is me and so I figured I had nothing to lose but my time.
Things I liked:
* * Super easy installation. Create a mysql database, go to the web-based installer, point at database, verify settings, click next a few times and boom. Installed.
* 2-way Contact and Calendar syncing with Outlook. Probably not a huge deal for most Linode'rs but if you use Outlook at work and want a sync'ed calendar this is fantastic. * 3-column Layout. I prefer the three column layout (Folders | Headers | Message Body) so this was nice to have in here. You can turn this off if you hate it. * Multiple Account Integration. I can set it up so it pulls and sends mail from any mail server not just my local setup. Neat. * Powerful and easy to use filter rule interface. * Generally just works. I found the whole installation, configuration, and usage really intuitive. There's no weird surprises or gotchas.</list>
Things that aren't so hot:
* * The interface is certainly usable but how it looks out of the box is not my favorite. I didn't care for any of the other supplied skins much better. YMMV.
* No Threaded Conversations. This is my biggest complaint. I really wish I could arrange my emails by thread.</list>
So that's it really. Overall I'm pretty pleased! I hadn't heard of this thing before today so I thought I'd share it. If you're looking for a RoundCube that's a bit more capable this is worth checking out.
I'm pleasantly surprised how nice it looks, and how easy it was to install. I'm just a bit worried about how secure it is (or isn't). Other ajax webmail clients (I'm looking at you, roundcube) seem overly prone to XSS and other vulnerabilities.
Update: It doesn't handle large mailboxes very well though. Takes about 30-40 seconds to check for new mail on my 900MB mailbox, via courier IMAP with maildir backing.