Twitter4R v0.2.0 now provides 100% Twitter REST API coverage for Ruby. This is the open source project providing Ruby bindings for the Twitter REST API. Most appropriate for pure Ruby and Ruby on Rails projects.
The non-profit built for "opening America's government" has created an app to show off its Congressional data, as well as its API (our Sunlight Labs API profile). You can't get this app on the web, though, as it's available exclusively for the Android phone platform.