If you run a website that lets users submit any information (either to you, to other users or as part of a registration process), you've probably had to deal with spam and related issues.
One of the most popular ways of dealing with these issues is the use of CAPTCHA systems.
What is CAPTCHA? If you've ever been asked to type in a bunch of distorted letters and numbers before submitting something online, you've used a CAPTCHA system before.
The purpose of CAPTCHA is simple - it's difficult for a machine to read what the letters and numbers are so by challenging a user to do so, you can help eliminate submissions from bots and other automated processes.
Adding a CAPTCHA system to your website or application may have some value.
Here are some CAPTCHA tools:
reCAPTCHA kills two birds with one stone - the CAPTCHA challenges it offers up are sourced from scanned books that are being digitized by Carnegie Mellon University. Since OCR software isn't perfect, the CAPTCHA challenges users are asked to solve help books get digitized.
Securimage is a free PHP CAPTCHA script that is customizable and even generates audio CAPTCHAs.
BotDetect is a CAPTCHA solution for websites and applications developed with ASP and ASP.net
CAPTCHA for Perl
This script produces a CAPTCHA image for Perl scripts.
This hosted CAPTCHA solution can be implemented with any programming language and framework.