U.S. Patent and Trademark Office- Providing high quality and timely examination of patent and trademark applications, guiding domestic and international intellectual property policy, and delivering intellectual property information and education worldwide.
Google Patent Search- The power of Google applied to U.S. patents.
FreePatentsOnline.com- offers many of the same search features as the USPTO site, but with an easier-to-use design and with the patents displayed as PDFs. The PDFs allow downloading or printing of the whole document. Create an account to best utilize the site.
Patents.com- New as of late 2008, a patent database with an IP exchange portal. U.S. and European patents are promised; search options are still under development.
Pat2Pdf.org- Free patent search tool.