I created a way to filter junk using regular expressions in Mail.app. I've detailed my solution on this page.'
[robg adds: This free solution uses Python and AppleScript to add regular expression filter to Mail's junk filter, thereby allowing it to catch re-written words such as "v1agra." The page even includes a nice introduction to regular expressions, explaining how to add your own pattern definitions to the junk filter files.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031114152301353