The U.S. Federal Government has launched Data.gov.
Here's an excerpt from the home page:
The purpose of Data.gov is to increase public access to high value, machine readable datasets generated by the Executive Branch of the Federal Government. Although the initial launch of Data.gov provides a limited portion of the rich variety of Federal datasets presently available, we invite you to actively participate in shaping the future of Data.gov by suggesting additional datasets and site enhancements to provide seamless access and use of your Federal data.
Read more about it at "Data.gov Launched by Federal Government"; "Data.gov Launches to Mixed Reviews"; and "Data.gov Now Live; Looks Nice But Short on Data."