Google Patents is a search engine from Google that indexes patents and patent applications from the United States Patent and Trademark Office (USPTO), which are taken from the original USPTO database (which is in the public domain). All 8 million patents have been put in the database. Optical character recognition (OCR) has been performed on the pages to make them searchable. This searchability includes all US patents and published patent applications.
Since August 2012, European patent documents are accessible as well.
Read more about Google Patents: History and Background, Early Review and Criticisms