Since I need to access my e-mail from multiple platforms on several different computers (work, home, computer lab, library, etc), I use a web-based client called SquirrelMail.
However, you may be less interested because it requires a working installation of Apache and PHP, which isn't much of a hassle to do, but probably wouldn't be worth it if you were just using those for SquirrelMail.
Yeah, I like my SquirrelMail.
http://www.squirrelmail.org