It's ActiveSupport::Cache::MongoStore -- a MongoDB-based provider for the standard Rails cache mechanism. With an emphasis on fast writes and a memory-mapped architecture, Mongo is well-suited to caching. This gem aims to give you what the ubiquitous MemCacheStore does, but with Mongo's persistence. (And without having to put a second RAM devourer in an environment already running Mongo.)
Gemfile gem 'mongo_store', '~> 0.3.0'
install gem install mongo_store