Simple Captcha: Use Random Number
Here is an idea, use a random number as a captcha. Store the random number in a session variable and then have the user enter that random number in manually.
Why it works
Most bots (any?) won’t be able to recognize the system because the random number is plain old page text. The user has only a very simple task to perform, and the number changes every time the form is accessed, so even if the spammer submits manually, it’s labour intensive to keep re-entering a random number. It would be even more so when combined with techniques such as permitting only one submission every 30 seconds. Those with javascript enabled will never see the server side response, and those without it (such as Braille readers) will still get a meaningful message.
http://evolt.org/simple-captcha
Tags: captcha, coldfusion, programming