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A ‘Captcha’ (completely automated public Turing test to tell computers and humans apart) is a challenge-response test most often placed within web forms to determine whether the user is human or a spam bot. The purpose of CAPTCHA is to block form submissions by spam bots, which are automated scripts that post spam content everywhere they can.It usually involves an image showing some distorted text, requiring the user to type in what they see in the distorted text.

The original CAPTCHA examples are now trivial for current algorithms to recognize, and the only option that developers had, was to increase the complexity of the distortion. Successive CAPTCHA systems have added more distortion, extraneous lines and shapes, fuzz on the letters, multiple colors and different sizes all in an attempt to stay ahead of the spammers.

1.Recaptcha

reCAPTCHA is a free CAPTCHA service that helps to digitize books, newspapers and old-time radio shows.reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher. More specifically, each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA. This is possible because most OCR programs alert you when a word cannot be read correctly.

2.Akismet

Automatic Kismet (Akismet for short) is a collaborative effort to make comment and track back spam a non-issue and restore innocence to blogging, so you never have to worry about spam again.

3.Textdisguise

Text Disguise saw this problem that plagued many sites and went in search of a solution. The solution came in the form of CAPTCHA-images. We’ve already tackled the difficult task of image generation and distortion. And, with security in mind – we’ve developed an API that never actually transfers the word to your server, or to the user’s browser – eliminating any fear of someone/something listening in on the conversation and avoids the pitfalls associated with trying to encrypt the word and persist it on the client-side.

4.Defensio

More than just another spam filter, Defensio also eliminates malware and other unwanted or risky content to fully protect your blog or Web 2.0 application. For an example of this phenomenon, please see Barack Obama’s Site Leading to Trojan. This is a great example of malicious content that has been detected by Defensio.

5.Mollom

Mollom is a web service that helps you identify content quality and, more importantly, helps you stop spam on your blog, social network or community website. When site moderation becomes easier, you have more time and energy to interact with your community.

6.Notequalsoft

Notequalsoft is a small software development studio specializing in Microsoft® .NET™ applications and web development. Most software available on this site has been designed during the free-time of our developers, so please contact them to purchase.

7.asirra

Asirra (Animal Species Image Recognition for Restricting Access) is a HIP that works by asking users to identify photographs of cats and dogs. This task is difficult for computers, but our user studies have shown that people can accomplish it quickly and accurately. Many even think it’s fun!

8.opencaptcha

Basic installation is cut-and-paste, and requires no ability to program image manipulation scripts.New fonts, image algorithms, and distortions applied weekly.

9.captcha

Features:

• 50 different Captcha algorithms
• Audio Captcha support
• Custom Captcha image size
• JPEG, GIF, PNG and BMP Captcha image formats
• Custom Captcha code length
• Choose between alpha, numeric and alphanumeric Captcha codes

10.captchas

This server provides CAPTCHA images and audio files you can use in html-forms. CAPTCHA images can prevent forms to be used by robots.

11.captchator

• works with any web programming language and framework (PHP, Ruby/Rails, Python, ASP, Perl, …)
• works on any server and shared hosting account, does not require extensions or special software
• can be implemented in just a few lines of code
• is easy to integrate with existing apps/websites.

12.ironclad-captcha

There thousands of robot programs that surf the Internet and fill in the forms to promote their sites, post unsolicited advertising or even compromise web sites they visit.Here this is the Professional free CAPTCHA for your site.

13.phpcaptcha

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. Securimage does everything from generating the CAPTCHA images to validating the typed code. Audible codes can be streamed to the browser with Flash for the vision impaired.

14.protectwebform

Spam is not delivered manually. There are many so-called spam robots (bots) which are spidering the web in order to find the form. Whenever such robots find the web form it tries to post a spam message to it. The only solution to protect your form is to add a verification image. This method will not protect you from spammers that are delivering their spam manually, but it will stop all the spam bots. Statistics show that spam delivered manually is less than 0.01% of all web form spam.

15.PHP captcha script

freeCap is a GPL CAPTCHA script to stop spam. It was written as a proof-of-concept at a time when there were really no other good PHP captcha scripts around (2005). For a long time I think it’s not untrue to say freeCap was the best PHP CAPTCHA there was.

16.snaphost

You don’t need to be a programmer to implement the script. Just insert a few lines of their code into your HTML form.

17.spamerator

Spamerator was invented to help prevent robots from brute-force login cracking and spammers from automatically submitting garbage content to websites.

18.Vidoop

Intuitive and Secure Image-Based Authentication Solutions.

19.webspamprotect

WebSpamProtect allows you to instantly add verification image (CAPTCHA) to your web site and protect your forms against spam robots.