Search trademarks and patents here:
http://www.uspto.gov/
--I use this as an initial search and then turn it over to lawyers.
Census facts here:
http://factfinder.census.gov/servlet/BasicFactsServlet
--Always get great market research info here.
Library of Congress Reference shelf here:
http://www.loc.gov/rr/askalib/virtualref.html
--Have not used this one yet, but looks good.
Happy web fun!
worksdev