SignatureProfiler is a plugin for Apple's Mail application that adds missing features from the way that signatures are handled and managed with your outgoing mail messages, especially if you have multiple mail accounts.
(commercial) SpamSieve gives you back your inbox by bringing powerful Bayesian spam filtering to Mac e-mail clients. It's quick and easy to control SpamSieve from within your mail client, and you can customize how it interacts with the rest of your message sorting rules.
spamtrainer was written to assist mail services administrators in updating and maintaining their SpamAssassin bayes database. When run, it will read the designated HAM and SPAM mailboxes, update the SpamAssassin databases and delete mail that has been learned from.
SpeedMail is a speedup Mail.app.
SweetMail is an e-mail program that supports multiple accounts and threading.
(shareware) ThinkArch allows you to archive MS Entourage email messages in DEVONThink Pro.
UTF-8 Output support for Pine
A small patch to enable UTF-8 output in Pine.
Webmailer is a program (actually a preference pane) for Mac OS X which lets you set it to open any URL or run any shell script, complete with the parameters lifted from the mailto link.