ChicagoLobbyists.org is a great example of what can be done with open data: create a Web app that increases governmental transparency and informs citizens about a key aspect of contemporary governance.
From the creators:
What is this?
ChicagoLobbyists.org is an open data, open government, and open source project intended to improve the transparency of interactions between the City of Chicago and lobbyists and their clients. All data comes from the City of Chicago Data Portal.
Who built it?
This is a project by Chad Pry, Derek Eder, Paul Baker and Ryan Briones. It started on July 16th, 2011 at the Google Hackathon with some amazing help from Chirag Patel, Ruthie BenDor and Nick Rougeux. Paul noticed the lobbyist data, conceived the project, and got the team together. Chad and Ryan did the bulk of the backend development with help from Derek and Chirag. Derek and Ruthie did the frontend development. Nick and Paul worked on the user interface prototypes.
Would be great to see this on a state by state and federal level.