Mail.app.
While it may look like yet another flashy Apple application, it's really a very fast and intuitive mail program. Ignore the whole "look, we've got cool mail templates" bit, by the way. There are also lots of add-ons and extensions for it.
With POP, if you don't leave a copy of a message on the server, you can only download it to one client before the server removes it.
Yeah, I tell it to leave messages on the server unless they're older than 2 weeks. It works very well that way, but IMAP is better.