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Public Access (Free) Patent Websites:
The easiest is to search is Google Patents, which has the most efficient search engine. Next is Patent Lens, where you can view foreign, US patents and patent applications. The U.S. Patent and Trademark Office search engine is less user-friendly. However, once you have determined the likely classification number(s) for your patent, The U.S. Patent and Trademark Office site is the most authoritative for searching by Patent Class.
USPTO: Search for Patents
United States Patent and Trademark Office. Instructions on patent searching strategy.
Contains all U.S. patents, U.S patent applications, and European patents. Older patents include many optical character recognition errors.
Scroll down past Scholarly works to the Patents section. Contains US patents from 1976+, European, World, and Australian patents. Contains patent applications from U.S., World, and Australia.
If you know the patent number, this is a fast way to download a patent. Limited to 5 downloads in one 24 hour period.
Espacenet offers free access to more than 70 million patent documents worldwide, containing information about inventions and technical developments from 1836 to today.
Other Patent Sites - Not Free