pocomail isn a nice program, but i found it a little buggy, sometimes the softwares filtering system would lose emails for no good reason, you also have to pay extra for scripting which gives features other clients have for free. It would be very nice if it were free, but its not (unless you happen to know a little chinaman in a long coat)
foxmail
alot of people like this client, but personally i thought it sucked. Maybe it just didnt run well on XP but it gave me shitloads of error messages. The program is developed by a chinese guy and support in english is a bit second rate. I would give this one a miss.
pegasus
is a nice, free but fairly basic email client. personally i just didnt like the interface and found the whole thing a little featureless.
the bat! is my favorite email client, it has a simple easy to navigate interface, is fast and unbloated and has has some powerful features, supports mulitple pop3 and IMAP accounts as well as PGP. This is shareware, unless you happen to know my friend Mr E. Mule