We've seen numbers that show that no matter how many web pages there out there, and no matter how much social networking stuff is available, the most-used facility on the Internet is plain old email. Even if you factor out all the spam and junk flying around, that's a lot of email.
Zimbra Desktop is a free email client with a lot of added goodies. It does all the email stuff you'd expect, allowing you read and write messages, and work with both POP and IMAP servers. It's also smart enough to work with web-based mail services like Gmail, Yahoo, and the lot. But along with email, it's also got contact management, calendaring, and more. Think of it as being like Outlook but without Outlook's hefty price tag. And since there are versions for Windows, Mac, and Linux, you're going to have to work pretty hard to find a machine you can't use it on.
You can download Zimbra Desktop for free. In addition, they've got big deal enterprise-size network email solutions (not free), although you don't need them to run the free app.