Pegasus Mail
Homepage Wiki
Email program with powerful filter and other features. Won't be developed any more since it's taking up too much of the developer's time, so get it while you can. Re-reading the wiki, it seems Pegasus will still be freeware for single users, but people using it for several accounts will have to pay for a licence.

Mailwasher
Homepage Wiki
Email is initially sent to your email server, where it stays until you download it to your computer. AFAIK, MS email software doesn't have any way of checking your email without actually downloading it from the server. Mailwasher gets around that, by downloading the headers of the emails on your email server, before you decide whether to download them or delete them. This can save both bandwidth and provide security by letting you delete suspicious emails before they get to your computer. NB. Pegasus Mail has these functionalities built in.

XNews
Homepage Wiki
Newsreader, of the Usenet variety. If you read newsgroups, this is the reader for you. Completely free, as the creator developed it for his own use and provided it for anyone else who's interested.

Buzzys.net
Homepage
Not a program, but a site reviewing and recommending freeware programs. Add it to your bookmarks now.