Hi Michael,
CapeSoft Draw has a Captcha function for doing this.
Incidentally, you may not need anything sophisticated. I was reading a post on codinghorror last night, and it was pointed out that his captcha is a fixed graphic (always the same graphic.) Apparently it's sufficient that the spam engines just move on. Coding Horror is quite a popular site, and it's working for him. My guess is that unless your site is specifically targeted a high-tech solution is not all that necessary.
Cheers
Bruce