The US Census Department provides the most comprehensive demographic information about America. Unfortunately, most of this data is difficult for researches to access.
As an example, here is a list of all American Community Surveys (ACS): http://1.usa.gov/1geFSSj. However, only surveys taken since 2010 are accessible via the Developer API (http://1.usa.gov/1nYk8OU).
This makes it extremely difficult to perform historical demographics anlaysis. As an example of research that this API enables, please see my choroplethr R package which creates thematic maps (choropleths) of Census data: http://bit.ly/1eZzNWP.
I would like to empower people to view demographic maps of any Census data from any year. That is only possible if the data is made available via the API.