Google Patent Search
Use Google Patent Search as an initial starting point for investigating patents--it covers up to those published in the last few months. Google Patents provides patents in PDF format. For a more comprehensive search, go to the USPTO or other patent offices, such as European Patents.
US Patent & Trademark Office
Complete listing of U.S. patents from 1790 to the present. The results include the fulltext of patents, as well as TIFF images. You can also search for patent applications since March 2001.