I found this really cool project. It is called SURBL or Spam URI Realtime Blocklist.
Instead of blocking messages based upon the computer that send the spam, it actually looks at the content of the message. Since the whole purpose of spam is for someone to make money, the spam messages generally contain urls for the receipient to buy what the spammer is selling. SURBL looks at the urls listed in the messages. If a lot of other people have reported messages that have the same url as spam to SpamCop, then this message will be considered spam.
SURBL web site