I have used qpopper for my pop3 needs and have been very satisfied with its features. I have used mainly sendmail with it.
http://www.eudora.com/qpopper/
If you have not decided on a MTA for the server, I would reccomend Exim. I recently had the priveledge to review an Exim mail server setup and I must say, it is far superior to the Sendmail system that is installed by default on most Linux systems. I also believe that Exim comes with a pop3 option that integrates seamlessly in to its mail system.
http://www.exim.org/
Another solution is if you want to host the email elsewhere, you could use fetchmail to get the mail from the remote mail server and then place it in the appropriate user mail files on the local server.