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

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

Recommendations for Ruby and Rails using collaborative filtering

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