Related link: http://www.spambouncer.org
I have mostly been away from my unix account on PANIX, so I have not been irritated by all of the spam that has shown up in my inbox. After the MyDoom incident, I started to get a lot of it. I think the spammers must have made a slight jump ahead in the arms race. Now they purposedly misspell words and construct much more natural looking messages. After a quick download, I no longer have to see those messages.
Every time a get a surge of spam in my inbox, I simply update my SpamBouncer filters. It uses procmail behind the scenes, and the author, Catherine Hampton, updates everything religiously. I do not have to fool with anything. Indeed, I can update everything with a cron job if I was motivated enough to take the five minutes to set it up.
As far as I know she has never asked for anything in return, and everything is available under the Free Software Foundation’s COPYING principles (which seems a little more vague than being licensed as such). Hooray for open source and huzzah for community!