Ransack is the successor to the MetaSearch gem. It improves and expands upon MetaSearch's functionality, but does not have a 100%-compatible API.

Squeel unlocks the power of Arel in your Rails application with a handy block-based syntax. You can write subqueries, access named functions provided by your RDBMS, and more, all without writing SQL strings. Supporting Rails 3 and 4.

Allows simple search forms to be created against an AR3 model and its associations, has useful view helpers for sort links and multiparameter fields as well.

Basepack for Rails

Model-wrappers to customize ransack search

Description of Uibox.