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

Anti Captcha is an automated CAPTCHA solving service

anti-captcha is a Ruby library for recognize captcha on anti-captcha.com

Simple gem to add captcha or negative captcha to your Rails 3 application

DeathByCaptcha allows you to solve captchas with manual labor

A captcha solver (only for easy ones) gem

A Google-style captcha for enterprise Rails apps

A Google-style captcha for enterprise Rails apps

SimpleCaptcha is the simplest and a robust captcha. Its implementation requires adding up a single line in views and in controllers/models. SimpleCaptcha is available to be used with Rails2.0 or above and also it provides the backward compatibility with previous versions of Rails.

A captcha verification approach for Rails apps, directly integrated into ActiveRecord’s validation mechanism and providing helpers for ActionController and ActionView.

ReCaptcha implements view helpers to generate ReCaptcha code, with the noscript counterpart, required methods that interface with the HTTP API for captcha verification, a DSL to generate a before_filter and the code to implement AJAX captcha validation.

Simple reverse captcha solution that consist in create a hidden field that should be always empty.

Captcha form Jike API

Simple, easy captcha for Sinatra

RMagickCaptcha is a gem that implements captcha for using in Rails application.

generate captcha by dalli,use the memcache

Gem that uses RuCaptcha API to solver any type of captcha

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

RedisCaptcha provides simple captcha that can be read by human.

MVC based Math, Logic Captcha