DataFabric: Easy Multiple DB Sharding Support for ActiveRecord
FiveRuns has released a library called DataFabric as an open source project on Github (with a MIT-esque license). DataFabric provides "flexible database connection switching" for ActiveRecord models, which permits easy replication-access and sharding support. DataFabric can be quickly installed with a simple gem install data_fabric
Rick Olson's Masochism is a pre-existing library that provides the multiple connection feature to AR already, but DataFabric builds on that by also providing support for sharding with a simple control method.