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.

Arithmetic Captcha

A Google-style captcha for enterprise Rails apps

ZeroCaptcha allows companies to solve captchas with artificial intelligence software, which is faster than humans and very accurate

A Rails captcha gem that attempts to determine whether or not a user is human.

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

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

A Google-style captcha for enterprise Rails apps

Drop in feedback engine with customizable fields, and captcha available

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

Captcha library which uses imagemagick to create captchas

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Forked from https://github.com/kares/simple_captcha

Description of RailsCaptcha.

Anti-spam protection for your website

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

Solve Media CAPTCHA Replacement

Simple captcha based on plain text questions.

This plugin adds helpers for the YinXiangMa CAPTCHA API

Simple gem to add captcha or negative captcha to your Rails 3 application