A number of websites offer a free patent search tool, including Free Patents Online (freepatentsonline.com) and the United States Patent and Trademark Office (uspto.gov). Free patent searches are a good way for inventors to conduct preliminary investigations into whether patents have been filed or granted on ideas or inventions similar to their own.
The United States Patent and Trademark Office allows users to access full-text U.S. patents issued from 1976 onward. It provides PDF images of U.S. patents issued from 1790 onward. Users can choose between a quick search, advanced search or a search based on a patent number. It is also possible to conduct searches on international patents from this website.