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

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

Rack middleware Captcha verification using Recaptcha API.

Rack middleware Captcha verification using Recaptcha API.

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.

Simple negative captcha for Rails

A Google-style captcha for enterprise Rails apps

Quick little arithmetic plain-text captcha

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

Simple negative captcha for Rails

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

Simple, easy captcha for Sinatra

A Google-style captcha for enterprise Rails apps

Merb plugin that provides resource access rate limits and captcha'ing

Merb plugin that provides resource access rate limits and captcha'ing

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.

A plugin to make the process of creating a negative captcha in Rails much less painful

BrainBuster - a logic captcha for Rails