“Google Patents covers the entire collection of granted patents and published patent applications from the USPTO, EPO, and WIPO. US patent documents date back to 1790, EPO and WIPO to 1978.” A Useful adjunct to the Google Patent Search is the Prior Art Finder which draws on the contents of Google Books and Google Scholar.
Find highly relevant patents quickly and accurately. PatentScout is an easy to use and private search tool that helps you discover relevant patents, organizations and technologies using semantic search or advanced keyword syntax. University-wide access to this resource is provided by the office of Industry Engagement and Commercial Venturing and requires email registration.
Coverage varies; World Intellectual Property Organization (WIPO)PATENTSCOPE database is for searching international patent applications(WO)and national collections primarily for selected countries in Asia, South America, Africa, and the Middle East. Graphing tools, linguistic search, browsing capability, sequence tracking are among the value-added features. [This resource is publicly available.]
"Espacenet offers free access to more than 80 million patent documents worldwide, containing information about inventions and technical developments from 1836 to today." It includes documents from the USPTO, the European Patent Office (EPO) and WIPO's PatentScope. NB: You can also search Espacenet in German and French.