Stop Spam!

http://www.scambusters.org/stopspam/index.html
Good list of other resources (links to web sites).

SpamCon Foundation: to reduce spam (junk email)

http://www.spamcon.org/
Organization trying to reduce spam with what appears to be a legislative focus.

Spam Site | Spam Filter Reviews

http://www.spam-site.com/
Lots of information on Spam and various tools that are available.

Anti-Spam Filters

http://mmmservices.web.cern.ch/mmmservices/Help/contents/0_4_spam-filters.asp
A nice explanation of the various filters being applied by one hosting service.
1. Checking the validity of the envelope sender address
2. Well known spam destinations
3. Attempts to use CERN as a relay are rejected
4. Well known dangerous attachments
5. Bad local addresses
6. BadStrings (viagra)

freshmeat.net: Category Reviews - Spam Filters

http://freshmeat.net/articles/view/964/
August 23, 2003 review and discussion of various Bayesian and heuristic filters.