Patents are rights granted to inventors to exclude others from using, making, or selling their inventions for a defined period of time. In the United States, the protection for utility patents is 20 years. The purpose is to stimulate innovation by granting inventors time to benefit from their inventions.
US Patent full-text databases - 1790 to date, full-text 1976 to date. Applications from 2001.
Google patents - full-text from 1790. Includes these countries (see codes)
Canadian Patents Database - Patents from 1920 to date. Includes images, bibliographic data and text.
Espacenet - 70 million patents world-wide. Video on YouTube.
Freepatentsonline - 1976 to date
WIPO (World Intellectual Property Organization)
Other international patent offices (listed on USPTO site)
Comparison of sites for searching patents (MIT - see table midway down page)