This is the search engine of the European Patent Office. The freely available database gives you access to 60 million patents (mostly full text) from all over the world. Searches can be limited to countries. The database includes patent applications and granted patents.

A patent search from the Irish Patents Office or from the UK Intellectual Property Office website will link you to this database.

This freely available database provides access to U.S. patents from 1790 to the present. Patents are available in full text from 1976 and as TIFF images from 1790-1976. Patent applications are available from 2001.

Patentscope is the search tool of the World Intellectual Property Organization (WIPO). It allows users to search patents and patent applications from different countries. While coverage is different, the extent of the coverage varies by country, e.g. timespans may vary, etc. Most patents are available in full text.

Google has its patent search integrated into the general search. You can search for a patent by title or author and then refine your results on the left hand pane to patents. Google Patents covers US patents, but recently added some European patents as well. Patents are offered in full text and an additional Prior Art Search lets you find related material from Google Books, Google Scholar and other sources.