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.

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.

A Ruby library for integrating Private Captcha verification into your applications

RedisCaptcha provides simple captcha that can be read by human.

Simple, easy captcha for Sinatra

This is a Captcha gem for ruby. It drawing captcha image with C code so it no dependencies.

A simple image-based CAPTCHA engine for Rails 6+, works offline and supports Devise integration

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

A simple and custom captcha for Rails3.

Rails mountable engine, that provides captcha support

generate captcha by dalli,use the memcache

MVC based Math, Logic Captcha

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

Simple captcha based on plain text questions.

simple stupid captcha

adds audio support to simple captcha

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

Simple Audio Extension to Simple Captcha

HumanDetector is a friendly logic captcha for Rails the doesn't require any external system dependency (e.g. external captcha API connections). It uses a simple question that any 7 or more years old human can understand, but that a spambot will not be able to answer. This Gem supports Ruby v1.9.2...

This is a Captcha gem for Rails Applications. It drawing captcha image with C code so it no dependencies.