Fight back against the spamers
This is a collection of things you can do to fight back against spam. This is NOT the same as blocking spam. There are plenty of tools out there to do that, but none of them cause any real damage to the spammers. This is a list of things administrators can do to HURT the spammers and reduce their effeciency in sending out spam.
Tarpitting
A tarpit is a fake SMTP server that accepts connections but holds the connection open for as long as possible, usually by responding very slowly. The benefit is that as long as a spammer is held up in your tarpit, that's all the less spam it's able to send to others. If the MTA being used to send spam will only send one message in the queue at a time, the impact of getting caught can be considerable. Add this to the fact that tarpits usually give a temporary error and tell the sender to try again, the spammers will often get caught multiple times before giving up.
More info:
Annoying spammers with pf and spamd - A guide on setting up a tarpit with OpenBSD.
Using statistics to cause spammers pain - An explaination of tarpits and a description of a Java tarpit proxy, TarProxy.
Poison pages
Poison pages are dynamic web pages that have fake, usually randomly generated, email addresses. There can also be links to other fake pages containing more email addresses. The idea behind this is that the spammers bots will find the page and add the addresses to their lists. This reduces the percentage of valid email addresses the spamers have, thus reducing their value.
More info:
Wpoison - Perl script that dynamically creates pages of email addresses and urls pointing to itself, in an endless loop.
spampoison.com - A page you can link to from your site that will take crawlers to a spam poison page.
Email URL crawlers
A URL crawler parses an email message for website links and loads them over and over, in an effort to waste the website's resources and/or bring it down. A single user doing this will have minimal effect, but there are some communities that band together to accomplish this.
More info:
Spam Slayer - A PCWorld article on Blue Security's Blue Frog software. (Update 20060516: Blue Security is currently down due to attacks by spammers. They may or may not return.)
If you know of more ways to fight back Let me know.