I like the invisible field suggestion and I'll have to try it. But I have also used a CAPTCHA solution that is very easy to work with. Even though all CAPTCHA presents useability issues, this one, at least, has an audio version that reads the image for the visually impaired and also has a reload button to generate another CAPTCHA image if the first was too hard to read. Check it out at ProtectWebForm.com. I wouldn't use it for secure information because it seems to run the captured inputs through their server then route them back to your server, but for your run of the mill inquiry form, it's okay. Another interesting version of CAPTCHA is found at ReCaptcha.net.