Preventing form spam

I was having problems with form spam on many of the web sites that I've created. I did some research and came across a web site which helps ALOT with preventing form spam. In fact, I haven't had any form spam from the sites that I've enabled this service on. The site is at http://www.projecthoneypot.org.

This site offers the following services

• Blacklisting lookup service (via dns query) which allows you to look up an IP address and get spam threat scores back from their database. The API is available at http://www.projecthoneypot.org/httpbl_api.php.
• Set up your own honey pot for each site. I think this is fine since spammers will visit your site anyway. So, you might as well divert them from the contact page (where your form or e-mail addresses are located) and give them a fake e-mail address that will be logged by project honey pot. This page has an FAQ about installing a honey pot on your site.
• There's more, but the ones above are the ones I use the most.