Dien,

You must be reading the same online publications...

ZIPskinny looked like quite an interesting concept... But the old data kinda kills it...

Here's a few other interesting links/resources that many know about...

But a couple are a little more up-to-date and contain all kinds of Useful information...

http://factfinder.census.gov/
http://quickfacts.census.gov/qfd/index.html
http://www.usa.gov/Topics/Reference_Shelf/Data.shtml
http://www.city-data.com/

geodata.gov is a web-based portal for one-stop access to maps, data and ... This is a U.S. Federal Government computer system that is "FOR OFFICIAL USE ...

It works fine for me... Use it "FOR OFFICIAL USE only...
http://gos2.geodata.gov/wps/portal/gos

Phil