Recommendify is a distributed, incremental item-based recommendation engine for binary input ratings. It's based on ruby and redis and uses an approach called "Collaborative Filtering"

A Like/Dislike recommendation engine for Ruby apps using Redis.

Fast and efficient recommendations and predictions using Redis

Recommendations for Ruby and Rails using collaborative filtering

An awesome, flexible, powerful, useful, tricky and liar Rails 3 recommendations engine.

Implementation of the weighted Slope One collaborative filtering/recommendation algorithm.

A ruby library to find similar objects and make recommendations based on activity of objects