1. USPTO Trademark Search Old style text-based trademark/application searching
2. WIPO Global Brand Database Internatinoal text-based trademark/application searching
3. DBA and Business Name Registration Search (just a few examples - note that individual states consider business names and trademarks differently (e.g., Illinois)):
4. GoDaddy WhoIs Meta Domain Name (URL) Search Consider the value of a trademark if the URL is not available
5. World Health Organization International Nonproprietary Names Pharmaceutical and biologic generic name parameters and search
6. Consider that common law (unregistered) trademarks might also exist!
NOTE: A DBA registration does not confer trademark protection for the mark. Also consider that many marks in the U.S are not provided on the Principal Register, but on the Supplemental Register. The International Trademark Association provides a great Fact Sheet for trademark registrations.
Several professional search firms are also available to coduct trademark searches, such as: