SpamAssassin is a great open source software package that provides bayesian spam filtering for email. It is very customizable, and has lots of cool 3rd party plugins for making your life as a server administrator just that much easier.
Lately I was configuring SpamAssassin to do some filtering and needed a way to test that the spam filters were being tripped. Without writing emails that were chock full of obscene words, I found the default text string that triggers an email to be flagged automatically as SPAM.
I know this will help some wandering server administrator out there somewhere!