ar_jdbc_pg_array includes support of PostgreSQL's int[], float[], text[], timestamptz[] etc. into ActiveRecord. You could define migrations for array columns, query on array columns.

ActiveRecord JSON serializer

Make left joins feel like first-class citizens in ActiveRecord.

Lookup tables using activerecord and array fields

Replace missing image files in your models with defaults.

Daemon for sending butches of emails via Amazon SES using ActiveRecord for storing messages. Handles daily quotas, max send rate.

Use marshal to serialize things, similar to ActiveRecord::Store

Memoize ActiveRecord objects and search there.

Useful to generate namespaced database models for a legacy database

Monocle helps you manage your DB views.

translation support for your models with an I18n active-record backend

A library which offers flexible, chained serializion for Active Record

Execute multithreaded code while still using transactional fixtures by synchronizing db access to a single connection

ar_mysql_flexmaster allows configuring N mysql servers in database.yml and auto-selects which is a master at runtime

A little gem that lets you execute lower-level ActiveRecord queries and access the resulting rows like you would by calling `find_by_sql`

ar_orderable adds a consistent way to order results at database level using ActiveRecord. Supports ordering by multiple attributes and by associated model's attributes.

You can order AR records and skip callbacks

Authorization Gem for Ruby and Ruby on Rails projects

Use a Postgres array column to manage a model's roles

Provides single sign-on authentication for web applications using the CAS protocol.