Jonathan Zittrain and Benjamin Edelman of the Berkman Center for Internet & Society at Harvard Law School are studying filtering, done voluntarily by Google and others. So far they have a list of 113 censored sites that don't show up on searches, mostly in Germany and France. There is a lot of the expected facist stuff, but also The Chinese Consultation Network, filtered from GR and FR versions of Google. Zittrain and Edleman are taking submissions.

CNET, the Crimson and the NY Times have all mentioned the study. Here is the Chilling Effects list of cease and desist letters to Google.