Social Explorer is a group of New York demographers who have taken year 2000 US census data and made it into interactive maps. And they've done a fantastically thorough job. You can zoom all the way from the national level to your own neighborhood (even the street you live on), and see all sorts of different data, from income to industry to gender to ethnicity to means of commuting to family structure. Want a map showing percent of foreign-born nationals who immigrated in the last five years? It's there. Want a map showing percentage of self-employed males? It's there. Percentage of housing where rent is between $600 - $800 per month, or where heat is provided by solar power? It's there. Populations of Estonian ancestry? It's there.
They even have a few animated maps showing population changes from 1910 to 2000, and they claim to have data going back to 1870. It seems you can only browse Y2K data, but you can create slideshows to morph from one map to another, showing correlations between different data (e.g. median income and high-school dropout rates).
There are a few problems here and there (individual pages don't have their own URL's, so you can't email someone a map; animations can be spotty; and their downloads page promises that a standalone version can be downloaded as of last week), but those are minor foibles in an amazing tool. You could spend hours, if not days, poking around in there. It has an enormous wealth of information.
