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

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 and Rails 3.0.x.

Captcha-Plugin for Rails

SimpleCaptcha is available to be used with Rails 3 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 captcha verification approach for Rails apps, directly integrated into ActiveRecord’s validation mechanism and providing helpers for ActionController and ActionView.

ActsAsTextcaptcha provides spam protection for Rails models with text-based logic question captchas. Questions are fetched from Rob Tuley's textcaptcha.com They can be solved easily by humans but are tough for robots to crack.

A Google-style captcha for enterprise Rails apps

A gem to generate captcha pictures.

Due to many unrequited complaints to the recaptcha team with regard to its unreadability for real human users, this gem offers a more readable selection of captcha images.

Captcha Client for YunPian.com

Use Turnstile by Cloudflare to perform captcha validation in your Rails app

Ruby package for easy integration with the API of 2captcha captcha solving service to bypass recaptcha, hcaptcha, funcaptcha, geetest and solve any other captchas.

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

Captchator as Rack middleware, and helpers for sinatra.

AntiCaptcha api ruby wrapper

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

BrainBuster - a logic captcha for Rails