Object geocoding (by street or IP address), reverse geocoding (coordinates to street address), distance queries for ActiveRecord and Mongoid, result caching, and more. Designed for Rails but works with Sinatra and other Rack frameworks too.
Official Geokit plugin for Rails/ActiveRecord. Provides location-based goodness for your Rails app. Requires the Geokit gem.
RGeo is a geospatial data library for Ruby. It provides an implementation of the Open Geospatial Consortium's Simple Features Specification, used by most standard spatial/geographic data storage systems such as PostGIS. A number of add-on modules are also available to help with writing location-based applications using Ruby-based frameworks such as Ruby On Rails.