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Google Patents
Covers the entire collection of granted patents and published patent applications form the USPTO (1790 - ), EPO and WIPO (1978 - )
Derwent Innovation Index
Access over 14.3 million basic inventions from 40 worldwide patent-issuing authorities (including backfile data to 1963 + full text)
DEPATISnet
Access the database of the German Patent and Trade Mark office which contains about 30 million documents from all over the world (including the full text of all german patent documents since 1877).
esp@cenet
Access the database of the European Patent Office (EPO) which contains about 59 million patents from 72 countries.
US Patent Office (USPTO)
Access the database of the USPTO which contains issued patents (full-text since 1976, full page images since 1790) and published applications (published since 2001 march 15th)