Demographic Data By Zipcode
Demographics on education, income, race, occupation, marital status, etc. are available by zip code from the Zipskinny website. You can also view various charts and maps on zipcodes, and can compare up to 20 different zipcodes. Note: The zipcode data from Zipskinny are based on the 2000 Census Bureau data, so might be a little bit outdated, but it still seems like a useful website for basic demographic information by zipcode.
(HT: Grant Leonard)