Here you will find a variety of patent search tools available for your use.  

USPTO Patent Search

This is the definititve search engine for finding information on patents issued in the U.S.  Provided by the USPTO for free this site allows users to search patents issued after 1976 and has recently added a limited search for applications.

Patent Scope

Provided by the World Intellectual Property Organization (WIPO), Patent Scope is a free search engine that covers Patent Cooperation Treaty applications housed at WIPO.

esp@cenet

Search service for the European Patent Office that also provides searches for the EU, EC, and the individual national offices thereof.

Google Patents

Google's version of patent search.  Allows for a flexible patent searching for any patent, however, issues have been reported that certain patents are difficult to find due to OCR errors.  

Free Patents Online

Free patents online provides a free search service that covers the U.S., WIPO, Japan (abstracts only), and European patents.