The United State Census Bureau site is great resource for quick statistics, but often some expert analysis would be helpful in getting the exact information you are looking for.
The Census Library provides Publications, briefs and reports from Census Bureau experts based on the raw data provided by the Census.
American FactFinder also displays Census data in more user friendly way by comparing data from several censuses and surveys and combining them into tables that can make for easier analysis. These tables are aluable examples of grey literature.