A government-wide search and notification system. Currently deployed to scout.sunlightfoundation.com
Sinatra is used for the web framework, MongoDB for the database, Postmark for email delivery, HTTParty for API consumption.
The architecture is conceived of as a series of tubes that receive search result data from remote APIs that users can subscribe to for updates. Each type of data that comes into the system (bills in Congress, federal regulations, etc.) is defined a set of small adapters to consume that data and display it in a variety of places.
We'd really love it if others used the Scout codebase to set up their own alert system. To that end, Scout's architecture is fairly well decoupled from the specific data sources that Sunlight's implementation currently uses.
But if you do want to set this up yourself, there will surely turn out to be more to do! Send konklone a message if this is something you're interested in.