Corporationwiki.com - Provides information about certain corportations.
Domaintools.com - Whois and reverse whois search. Find the owner of a domain.
Zabasearch.com - Find public listings of individuals and companies.
Switchboard.com - White and Yellow page listings. Link to other databases.
Danhatesspam.com - Links to every secretary of state for looking up corporate info.
Knowx.com - Lets you search all corporations at once.
Download.com - Sam Spade is invaluable.