The University of Washington Engineering Library is a designated Patent and Trademark Resource Center. For searching assistance consult their Patent and Trademark Information Guide.
Patentscope - World Intellectual Property Organization (WIPO)
Global Patent Search Network - Chinese patent documentation; A cooperative effort between the USPTO and the State Intellectual Property Office of the People's Republic of China
Google Patents - indexed patents and patent applications from USPTO, EPO and WIPO
The Lens - global open source open access patent search
Why search patents?
MIT Libraries video tutorial "Patents: An Introduction - Why search for patents? What is a patent?" - a brief 5 minute video explains why a patent search is an important part of your research