SimpleCaptcha is available to be used with Rails 3 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!

Captcha generator for rails

Arithmetic Captcha

Arithmetic 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.

Define questions and answers in a YAML file and validate your models with a single line of code. No more captchas or other stupid tricks.

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

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

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

Simple negative captcha for Rails

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

BrainBuster - a logic captcha for Rails

A Google-style captcha for enterprise Rails apps

A Google-style captcha for enterprise Rails apps

dongsx

This gem enable to use Captchme. Anti spam, Monetize captcha and Audience qualification

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

Quick little arithmetic plain-text captcha