Google Patent Search

Google Patent Search is a new tool that searches the full text of the U.S. patents. "Google Patent Search covers the entire collection of patents made available by the USPTO — from patents issued in the 1790s through those issued in the middle of 2006."

The application has a similar interface for browsing PDFs with Google Book Search. You can search within a patent, view citations and references. There's also an advanced search that lets you enter a patent number, a title, an inventor, an assignee and issue/filing dates.