Here’s a list of 34 free Captcha scripts and anti-spam services you can use to provide protection from spam bots and ensure that only humans perform certain actions. Take note that using any of these Captcha scripts and services doesn’t guarantee that your 100% free from spam.

Hosted Anti-Spam Services

1.) reCAPTCHA

reCAPTCHA is a system developed at Carnegie Mellon University that uses CAPTCHA to help digitize the text of books while protecting websites from bots attempting to access restricted areas. It is easy to integrate with wide support for programming languages and ready to download plugins such as for WordPress, vBulletin, Drupal, etc.

2.) Automattic Kismet

From the creator of WordPress, Akismet is a service to stop comment and trackback spam. When a new comment, trackback, or pingback comes to your site it is submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down.

3.) Defensio

Defensio is a spam filtering web service that you can use to protect your blog or web application from comment spam.

4.) Mollom

Mollom helps you keep your site clean and on topic by automatically blocking comment form spam, contact form spam and fake user accounts. We use a novel filtering technique based on the combination of content analysis and CAPTCHA challenges.

5.) VidoopCAPTCHA

VidoopCAPTCHA is an image-based verification solution used to distinguish a human from a computer program, protecting web sites against malicious bot attacks. When compared to traditional text-based CAPTCHAs predominant on the web, VidoopCAPTCHA is easier on the user.

6.) WebSpamProtect

WebSpamProtect.com 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, our system requires that you install a small piece of code onto your web page. Our code requires that your web site support PHP, Perl, ASP or ASP.NET.

7.) Microsoft Research – ASIRRA

Asirra is a human interactive proof that asks users to identify photos of cats and dogs. It’s powered by over three million photos from our unique partnership with Petfinder.com.

8.) Ironclad CAPTCHA

Ironclad CAPTCHA is rendered in realtime from 3D scene, where each object can be positioned, scaled, rotated and colored in random way. We then require user to count the number of three given types in the picture to pass the test.

9.) Protect Web Form

ProtectWebForm is a service to add CAPTCHA images and audio files on your website. You have to create an account, generate your CAPTCHA type with all the desired parameters and include it on your web pages.

10.) OpenCaptcha

OpenCaptcha is a captcha service which requires no special configurations or modules and requires no ability to program image manipuation scripts. New fonts, image algorithms, and distortions applied weekly.

11.) captchas.net

captchas.net provides CAPTCHA images and audio files you can use in html-forms. To use this technology, your web pages have to be generated dynamically in any programming language PHP, ASP, Perl, Python, JSP, Ruby.

12.) Spamerator

Spamerator was invented to help prevent robots from brute-force login cracking and spammers from automatically submitting garbage content to websites. It does this by sending an image with a random alpha-numerical code that only humans can see (not most robots, yet).

13.) Text Disguise

14.) Captchator

15.) Notequalsoft Captcha Service

PHP Captcha Scripts

1.) Secureimage PHP CAPTCHA

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.

2.) VeriWord

Veriword is a PHP Captcha system thats generate a question in form of an image with random baffle text or in another form such as an animated flash. The class is capable of picking a random word, generate captcha and store the word in an encrypted session variable for posterior validation eventually in a form on which it is used to prevent robots to have automated access.

3.) Advanced Textual Confirmation

Advanced Textual Confirmation is an universal antispam for forums, blogs, guestbooks, contact forms, and others. It is a smart textual CAPTCHA, which challenges site visitors only once, and then disappears.

4.) Cryptographp

Cryptographp is a PHP script for generate captchas. Cryptographp limit the robots bombarding spams and automating the forms: spaces members subscriptions, guestbooks, forums, etc. There’s also a plugin version for WordPress, wpMu for WordPress, Symfony and Guppy.

5.) 12 PHP Captcha Scripts Worthy To Mention

The following PHP Captcha scripts may either discontinued in its development, were written a long time ago, recommended on using hosted anti-spam services such as reCAPTCHA, don’t have much security compared to what was mentioned above,  few information is available on their website, authors didn’t respond to comments, don’t have a proper revision history or simply the output (verification image or text) look like the same as other Captcha system.

• Captcher
• HKCaptcha
• White Hat Captcha
• Visual and Audio PHP CAPTCHA Generation Class
• freeCap
• identiPIC
• Captcha ZDR
• Animated Math Captcha
• SOLMETRA FormValidator
• MathGuard PHP Form Anti-Spam Protection
• Animated GIF Captcha
• Psyrens AJAX Captcha

ASP Captcha Scripts

1.) Web Wiz CAPTCHA

Web Wiz CAPTCHA is free security image software written in ASP that displays a security image on your websites forms which only humans can read. This prevents your websites forms from being spammed by robots and hacked using dictionary or brute force hacking.

2.) BotDetect ASP Captcha

Lanapsoft BotDetect is an awarded website security solution designed to prevent automated submissions of ASP forms with CAPTCHA images. It’s a a COM Component that creates and validates image and sound CAPTCHAs, which can be simply added to your ASP forms using our ASP wrapper CAPTCHA module. (Registration required to download)

3.) Classic ASP Security Image Generator

Generate images with letters and numbers to make a CAPTCHA test. Completely free classic ASP VBScript. The secure code is completely random, dynamic image processing, no image file requires, no components requires, create a cross browser compatible BMP images and you can use a custom color palette for image.

Bonus

1.) Fighting Form Spam With CSS

An idea and tutorial for fighting form spam attacks with CSS.

2.) PWNtcha – Captcha Decoder

PWNtcha stands for “Pretend We’re Not a Turing Computer but a Human Antagonist“, as well as PWN capTCHAs. This project’s goal is to demonstrate the inefficiency of many Captcha implementations. Look at the bottom table where it listed hard Captchas.

You may post good CAPTCHA scripts or anti-spam services that we’ve missed so that we can add it to our our list, with credits (link to your website) given to you.