RedisCaptcha provides simple captcha that can be read by human.

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

Simple, easy captcha for Sinatra

Simple captcha based on plain text questions.

Captcha form Jike API

simple stupid captcha

adds audio support to simple captcha

A simple and custom captcha for Rails3.

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

Gem that uses RuCaptcha API to solver any type of captcha

Simple Audio Extension to Simple Captcha

A simple image captcha gem

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...

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

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

Rails mountable engine, that provides captcha support

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

MVC based Math, Logic Captcha

It's a Rails Captcha Engine for Rails 3 applications.