Each database has unique features and you might have one particular need or database search preference:
Espacent - International patent database containing 60+ million patents from more than 70 national and regional patent offices.
Free Patents Online (has patent maps)
Patent Lens - Australian, European, U.S. and World Intellectual Organization (WIPO) patents from 1970's to present.
PatentScope - (WIPO) International patent applications from 1978 to present.
Canadian Patents Database - (CIPO) Canadian patent records, 1869 to present; full text documents, 1920 to present.
Comparison of Free Patents Databases Online - with advantages and disadvantages of each - complied by PTRC Librarian Jim Miller.
Finding international patents involves using the Cooperative Patent Classification (CPC) system. This is a bilateral system which has been jointly developed by the EPO and the USPTO. It combines the best classification practices of the two offices.
This classification system is much like that used by the USPTO (USPC - United States Patent Classification) in that each invention is categorized into a particular set of classification terms. The classification term is made up of 4 four parts:
Section Symbol (Letter) - Class Symbol (2 digit #) - Subclass (letter) - Group Number (1-3 digit #)