U.S. Patent and Trademark Office
Database of U.S. patents available to the general public, including full-text patents since 1976 and full-page images since 1790.

Google Patent Search
Search U.S. patents from the 1790s to mid-2006 with the simplicity of Google's search engine. PDFs available.

European Patent Office
Database available to general public: includes European, world and Japanese patents; full-text and images for than 38 million patents from 71 countries; years covered vary from country to country.

Scirus
Scirus, a search engine that searches the Web for only scientific information, indexes approximately 13 million science or science-related patents from the United States Patent and Trademark Office, the European Patent Office, the Japanese Patent Office, and the Patent Cooperation Treaty of the World Intellectual Property Organization. Links to the full-text of the patents are available. To limit your search to patents only, use the advanced search feature.