Census Maps and Thematic Maps
Census Interactive Mapping
SimplyMap is a web-based mapping application that lets users quickly create professional-quality thematic maps and reports using demographic, business, and marketing data for the United States
Social Explorer provides easy access to demographic information about the United States, from 1790 to present. Built more for creating instructional slide shows of census thematic choropleth maps. Coverage is extensive, but not even. Not made for downloading, but it is possible to build reports.
Also easy to use. Data is at the county level only. Not meant for downloads.