This resource replaces the now defunct American FactFinder. This is the U.S. Census Bureau's main vehicle for distributing U.S. census data, including economic censuses and surveys and annual population estimates. Population, housing, economic and geographic data can be found here.
Looking for data from a University? Most campuses have a centrally located office that tracks their data. Here is an example of BGSU's Office of Institutional Research. These offices are sometimes called different names such as Institutional Effectiveness. However, they may be willing to share datasets with graduate students.