The Spammer’s Compendium, a list of the techniques that spammers are currently using.
popfile, John’s free anti-spam program.
One of John’s recent presentations.
John also puts out a bi-weekly newsletter on spam and spam-fighting.
Interesting points: the hard thing these days is not filtering out spam, John says, but not filtering out good mail. When you purchase an anti-spam system, what really matters is not just the false-accept-rate, but the false-reject-rate.