5 Free CAPTCHA scripts and services for websites

A CAPTCHA is a program that can tell whether its user is a human or a computer. You've probably seen them — colorful images with distorted text at the bottom of Web registration forms somewhere. CAPTCHAs are used by many websites to prevent abuse from "bots," or automated programs usually written to generate spam. No computer program can read distorted text as well as humans can, so bots cannot navigate sites protected by CAPTCHAs.

Following the list of free CAPTCHA scripts and services for websites:

1. reCAPTCHA
reCAPTCHA is a free service which helps prevent automated abuse of your site (such as comment spam or bogus registrations) by using a CAPTCHA to ensure that only humans perform certain actions.

2. Securimage
Securimage is an open-source free PHP CAPTCHA script for generating complex images and CAPTCHA codes to protect forms from spam and abuse. It can be easily added into existing forms on your website to provide protection from spam bots. It can run on most any webserver as long as you have PHP installed, and GD support within PHP.

3. WP Captcha-Free
WP Captcha-Free blocks comment spam by using a combination of time-based hash (a.k.a. Time Based Tokens, TBT) and JavaScript (AJAX). When a comment is posted the plugin validates a hash based on time (and some other parameters). Comments posted via automated means will not have a hash or will have an expired hash and will be rejected. Unlike using a captcha, this does not place any burden on the commenter.

4. Form-to-email script protected by Captcha
This form-to-email script is intended to collect data from an HTML form and send it to a specified email address using a CAPTCHA script to stop spam. It works with any kind of HTML form and you can use it multiple times on your pages.

5. WebSpamProtect
WebSpamProtect allows you to instantly add verification image (CAPTCHA) to your web site and protect your forms against spam robots. In order to protect your web form, this system requires that you install a small piece of code onto your web page. The code requires that your web site support PHP, Perl, ASP or ASP.NET. Basic plan is free but you can buy a Premium, Advanced or Professional paln with a little expense.