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In the past days I received several requests from my readers to suggest some CAPTCHA scripts and services quickly to embed into a web page. This post proposes a list of ten interesting scripts and services you can use to provide protection from spam bots and ensure that only humans perform certain actions.

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. Securimage does everything from generate complicated CAPTCHA images to making sure the code the user entered was correct.
3. 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.
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… This script is free and does not use any database. It is compatible with PHP >= 4.3.0. Cryptographp also exists into plugins for WordPress, wpMu for WordPress, Symfony and Guppy.
5. CAPTCHA-Service
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.
6. 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.
7. ProtectWebForm

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.You can also use it on your WordPress template with this plugin.
8. ProtectWebForm
OpenCaptcha is a simple web service which requires no special configurations or modules. Basic installation is cut-and-paste, and requires no ability to program image manipuation scripts. New fonts, image algorithms, and distortions applied weekly.
9. 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.
10. freeCap
freeCap is a GPL CAPTCHA script to stop spam. It has been used on any form on any kind of website. It does require some knowledge of PHP to install, though there are several captcha plugins for forum and blog software listed below.