Here is a series of U.S. Google Maps searching tools from Maps.Huge.Info that you might find useful:
Zip Code Boundary Map:
Town/City Boundary Map:
County Boundary Map:
Core Based Statistical Area (FIPSCSA) Boundary Map:
These are extremely easy tools to use - Simply click on the Google Map to map the boundary type or enter your information into the search field first. Maps.Huge.Info also has a nifty geocoder tool that lets you translate any U.S. address to latitude and longitude.
Using these tools in conjunction with this great demographics mashup gives you a pretty powerful series of (free) mapping tools for finding census information about certain areas of the U.S. using Google Maps.