From: H-Online
The US Census Bureau has made an HTTP-based API available which gives developers access to the data collected in the 2010 Census and in the American Community Survey from the same year. The terms of service attached to the API permit unrestricted use of the data as long as it is clearly attributed to the source and the application does not use the Census Bureau name to imply any kind of endorsement by the organisation. Furthermore, if the data is changed or misrepresented in any way, users may not claim that the source of the information was the Census Bureau.