you will get arithmatic sum with answer you just have to compare with ur user in controller

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

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

Captcha-Plugin for Rails

Wrapper for Captchabot.com

Simple negative captcha for Rails

A gem to generate captcha pictures.

Display and verify arcaptcha service captcha

Captcha Client for YunPian.com

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

Captcha form Jike API

Metabypass | Ruby-based easy implementation for solving any type of captcha by Metabypass

RedisCaptcha provides simple captcha that can be read by human.

A very simple rack captcha middleware via captchator.com API

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.

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

BrainBuster - a logic captcha for Rails