PATENTS
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Google Patents Search
Search U.S. patents from 1776 to present using keyword or patent number, inventor name, U.S. and international classification numbers as well as other limiting factors such as date.  Images are available without loading extra software.

Patents and Trademarks Office: Search patents and applications
Search patents and patent applications by keyword, number, classification and more from 1976 to the present.  Search patents before 1976 by patent number and classification number.

X Patents
Search all of the early X patents by the X patent number.

FreePatentsOnline
Provides fast, easy-to-use access by keyword or patent number search to millions of patents. Provides free PDF downloads. Includes a unique section called "Crazy Patents" that highlight the weird and wacky patents that have been issued.

www.pat2pdf.org
Free patent searching by patent number and downloading of patents in PDF format.