Xyggy Patent Search Tool

http://www.xyggy.com/

I came across the above website recently which does a fairly decent job of performing quick searching for similar patent information based on a given patent number or title. The search algorithm used seems to be a little more involved than simply text-based from my test searches. While the search interface does not allow field selection (i.e. assignee, date range, etc.), it does a pretty good job if all you are looking for is a simple tool to identify a listing of similar patents.