This is a searchable site from the U.S. Census. Using this site, put in a zip code, city, state, county, or town and find demographic information on the area you've chosen. You can view data in various forms: maps, tables, charts, and more. Most data results are downloadable.
This links to a search results page on Pew Research Reports. These reports are about different demographic topics in the U.S. It can be filtered by year and post type (i.e., blog or report). The site can also be used to generate new searches.
This site is great for a snapshot of an area. Enter in a zip code, and a list of "types" will emerge. Nielsen's My Best Segments operates on the idea that people want what their neighbor has and has created demographic "segments" of the American population. It's good for getting a sense of how the residents in a certain area may act (lifestyle, purchase types, etc.).