pg_search builds ActiveRecord named scopes that take advantage of PostgreSQL's full text search
The Brainstem gem provides a framework for converting ActiveRecord objects into a great JSON API. Brainstem Presenters allow easy application of user-requested sorts, filters, and association loads, allowing for simpler implementations, fewer requests, and smaller responses.
Use hashids.rb to store ActiveRecord IDs in URL non-obviously.
Asynchronously sends your ActiveRecord models for reindexing in elasticsearch by making use of tire and resque
Yet another hierarchy plugin for ActiveRecord using PostgreSQL's ltree
itunes-client provides a high level API (like ActiveRecord style) to control your iTunes.
An ActiveRecord plugin that denormalizes your existing ActiveRecord models