Search here for issued patents ("full-text since 1976, full-page images since 1790") or for published applications (since March 15, 2001). From the United States Patent and Trademark Office.
Search for European patents using this search interface from the European Patent Office. Available in English, French, or German.
". access over 75 years of patent descriptions and images. You can search, retrieve and study more than 1,500,000 patent documents."
From Georgia Tech, this page has links to patent databases, guides on how to search them and useful information about the U.S. Patent Office.