The Census Bureau now keeps only 2 censuses online. Currently, 2010 and 2000 are available online at the American Factfinder web site. Once the data for the 2020 census is available, the 2000 data will disappear.
Another source for information about the past censuses is located at the Census Bureau site. Under Measuring America: The Decennial Censuses from 1790 to 2000, you can find a history of the changes the Census has undergone.
The University of Virginia hosts a site which allows you to see the data from past censuses. You cannot download the data, but you can see the data in tabular form or create maps at the site. The site is called the Historic U.S. Data Census Browser. The maps created are in PDF format which you can then save.
Looking for census data by ZIP code? There are several options, but the Census Bureau does not control ZIP Codes. The U.S. Postal Service controls that geography.
ZIP Codes do not conform to requirements of census geography. ZIP Codes cross administrative boundaries and aren't always discrete entities. For sources of census data by ZIP Codes, click here.
For the 2000 Census, these are the files available:
Summary File 1 (SF1) = 100% count from the short form questionnaire.
Summary File 2 (SF2) = 100% count, short form questionnaire with detailed race and Hispanic or Latino categories, American Indian and Alaska Native tribes.
Summary File 3 (SF3) = Sample of the long form questionnaire.
Summary File 4 (SF4) = Sample of the long form questionnaire with data shown down to the census tract level for 336 categories of race, Hispanic or Latino, American Indian and Alaska Native and ancestry.