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

Implementation of captcha (Completely Automated Public Turing-Test to Tell Computers and Humans Apart) that is both easy to use and easy to customize/extend.

Antigate (Decode captcha service) wrapper for Ruby

A simple way to add honeypot captchas to Rails forms

A simple CAPTCHA implementation for Rails 5+ based on RMagick.

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

Death by Captcha Ruby implementation (HTTP API)

ALTCHA is a free, open-source CAPTCHA alternative that protects your website from spam and abuse

Integrate the AreYouAHuman.com CAPTCHA alternative human verification into your Ruby/Rails application

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.

Generates Text Catpchas, that are 'nice' by my own personal definition!

AntiCaptcha api ruby wrapper

Captcha library which uses imagemagick to create captchas

To help with keep the bots at bay this gem helps you easily add honeypots to forms and/or obfuscate your form field names

Helps humankind with simpler captchas

generate captcha by dalli,use the memcache

A simple and fast image captcha generator that uses opencv and memcached.

A honeypot style captcha extension for ActiveRecord. Simple one-line inclusion in the model combined with a simple form field should trap the bad guys in the molasses.

Simple Audio Extension to Simple Captcha

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.