Mineful.com offers web survey, forms and marketing analytics software and services, but they’ve also just added a new section with free demographic data tabulation and visualization for the United States, broken down by state, county, zip code, and many demographic parameters (age, education, ethnicity, employment, marital status, etc.).
Unfortunately, the data isn’t downloadable. But for a quick overview of basic demographic data, it does the job. Jaime Brugueras of Mineful, who sent me the link, says that they’re working on additional capabilities for the site like search, radius profile, and more geography types.