Yesterday I installed RoundCube Webmail on my host to try out. It's a:

…browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an e-mail client, including MIME support, address book, folder manipulation and message filters. RoundCube Webmail is written in PHP and requires the MySQL database. The user interface is fully skinnable using XHTML and CSS 2.

Since it's still in development, it lacks many features you might expect from other webmail packages, such as the ability to bulk move mail into a folder, or set up message filters. I have no doubt these features are coming though. But it's the super sleak, clean interface that makes it such a joy to use. Not too mention that the red and silver skin goes great with Opera.

At this point there is no user friendly installation, so you'll need to manually create your database for it to use, and modify some PHP files with the necessary settings. But anyone confident in making such changes isn't going to have any difficulty setting it up.

Unfortunately, the package cannot be restricted for your own use unless you put it in a password protected area. If it's publicly accessible, anyone can check their mail from your setup. Since I don't want to have to log into RoundCube then log into my mail I'm going to try modding it so it can only be used to check my own e-mail accounts.

Anyway, if you're interested in checking your mail via a web browser then I highly recommend you try it out. RoundCube makes Horde and SquirrelMail look like a pile of puke.