LoyalSimpleCaptcha is available to be used with Rails 3 or above and also it provides the backward compatibility with previous versions of Rails.

Russian Post tracking toolset, consisting tracking form bot (since Russian Post is too retarded for normal API) and captcha solver/recognizer.

Anti-spam protection for your website

Implements view helpers for the ASIRRA CAPTCHA system for Rails-based websites

Simple captcha image generator gem

A captcha that uses images instead of garbled text

Antigate (Decode captcha service) wrapper for Ruby

Captcha form Jike API

Drop in feedback engine with customizable fields, and captcha available

generate captcha by dalli,use the memcache

Solve Media CAPTCHA Replacement

Description of TinyCaptcha.

SimpleCaptcha is available to be used with Rails 3 + 4 or above and also it provides the backward compatibility with previous versions of Rails.

RubyGem package for visualCaptcha's backend service

A captcha solver (only for easy ones) gem

Captcha based on encyrpting and decrypting user data by mathematical operation

Ruby wrapper for Yandex.Cleanweb

API to interact with BrandCAPTCHA http://www.pontamedia.com/en/brandcaptcha/

Wrapper for Captchabot.com

Simple lib to generate a math_captcha to prevent spammers in form. Supports localization.