Lawrence Lessig, a well-known name in the open-source community because of his views and commentary on digital rights and privacy issues, has launched an (ambitious) effort to clean up Congress by using web-based grassroots efforts.
The website is called Change Congress, and aims to get candidates to commit to four pledges:
Don't accept money from PACs or Lobbyists
Get rid of earmarks
Increased transparency in congress
Public financing of campaigns
The four items seem like a good starting point, especially since earmarks have run rabid recently. To help track how well your congresscritter is doing, they have a helpful "sludge map" showing how much PAC money each district is taking in.