To help with keep the bots at bay this gem helps you easily add honeypots to forms and/or obfuscate your form field names

ALTCHA is a free, open-source CAPTCHA alternative that protects your website from spam and abuse

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

BrainBuster - a logic captcha for Rails

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