In response to a thread in one of the Apple fora, I've made a quick web page that tells in simple language how to use fetchmail to pull down your email so you can filter it with procmail, but still read it with the Mail application. This lets you use procmail's incredibly powerful filtering capabilities even if you aren't running a mail server. It will even work well with a dialup account.
[Editor's note: I can't say I've tried this one yet, but I hope to shortly. This looks like an easy to follow recipe for getting your own personal spam filter up and running.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2001121517274699