Check out qpopper from Qualcomm. Don't worry, it's free.
www.qpopper.org. They have very good instructions for setting up if you take the time to read them. Basically each user will need an account to get mail, but you can restrict the accounts by setting their shell to /bin/nologin.
If you want to use webmail, I think Squirrelmail is pretty much the de facto standard. It uses Cyrus IMAP, I think.
