PatentSurf

Patent Surf (http://www.patentsurf.net/) identifies itself as a discovery engine of U.S. Patents.

What does it do?
According to their web site, "This site empowers you to surf patents made available by the U.S. Patent & Trademark Office. As a starting point, please type a natural description of what you are interested in. Patents that are related to your description will be presented for your review. You may then surf (not search!) related patents, where you will discover interesting and unexpected relationships.There are currently 6,858,357,960,636 relationships in the PatentSurf.net database."

How do you use it?
You can use it only with natural language (apple pie). Boolean won't work (apple AND pie). Results are produced in a list or a grid. In a grid format, you can click on links Related Patent Hold Ignore. The Related link will re-focus your search around that patent. The Patent link will open the patent in a new window from the USPTO. The Hold link will change the background color and allow you to collect only those patents in a new window. The Ignore link will filter out those patents into a new window. In a list format, the list of patents is listed in the left column, with a view of the patent in the right column. The patent is in html format, from the USPTO.
In either "list" or "grid" format, you can sort the results by "relevance" or by "patent number."

This website is evolving and will allow you to search a patent by patent number, either from the search window, or by pasting the patent number at the end of link. For example, http://www.patentsurf.net/4,771,554.

Why use it?
It is pretty good in finding related patents, and fairly easy to use. At this time, there is no search fee.