Zipskinny is a great source for the most basic info of a zip code, any zip code. Born out of a hobby using Census 2000 data, Zipskinny points out that it should be looked at as a 'first glance' site only. My favorite is the zip code comparison function that lets you compare up to 20 zip codes.
Add to that Google Maps's Street View, EveryBlock, and Yelp and you have a nice little packet of research tools of neighborhoods. Nice if you wish to move - if you'd like to buy a house, try Trulia - and do a preview and great if you're doing preliminary market research of population.