If you run a web server, you probably have some forms on it, and if you have forms, you probably get spammed through them. Up until yesterday, I used to get 5 or 6 spam messages a day sent to me through my own press room contact form. How dumb are these people? The messages were all pretty much the same and all went to the same person--me. If I didn't click their stupid ad the first few times, why would I click it the 1000th time? The answer is probably that they never even looked at the form. They probably have a web crawler that finds online forms and just starts submitting spam to them, not worrying too much about whether its doing any good. Thus the beauty of the web being practically free makes way for the ugly. But you don't need a sermon on spam.
Yesterday, I wrote myself a captcha generator and added it to a number of my forms. As I usually do with these little scripts, I'm making it available for free download. It requires PHP with GD support for outputting GIFs, which any reasonably up to date PHP installation should have, and mySQL. Support for running Perl cron jobs is also a plus (you'll need to periodically dump unsolved captchas from the database--the solved ones get deleted automatically).