The European Patent Office's (EPO) patent search engine, Esp@cenet, searches published patent applications in over 80 countries. It is the most comprehensive free database available, with over 60 million patents. Some countries offer full text, others just citation and abstract.
Country Specific Patent Searching
Industrial Property Digital Library (http://www.ipdl.inpit.go.jp/homepg_e.ipdl)
The JPO (Japanese Patent Office) serves patents from their Industrial Property Digital Library. The PAJ (Patent Abstracts of Japan) database has word searching back to 1976. Their classification system (F1/F-term) search goes back to 1885. There is a three to six month delay in translations to English. The Japan Patent Information Organization (JAPIO) offers English-language access to Japanese unexamined patent applications and offers a machine translation of patents.
SurfIP, the Singapore government's patent database, has a structured search that accesses any or all of the following countries' patent information: U.S., China, UK, Canada, Taipei, Korea, and Thailand. The choices also include EP patents. Interestingly, Japan is not listed. The number of stars in the results list indicates perceived relevance, and an active link leads to the home site for the full document.
The Canadian Intellectual Property Office (CIPO) database has full patents back to 1920 with text searching for patents back to 1978. Earlier patents are searched by patent number, title, inventor/assignee, or classification.
Countries like Australia and New Zealand have put patent information online recently. Patent Analysis searches both along with the U.S. and European patents.
Google Patents (http://www.google.com/patents)
Google Patents searches only U.S. patents, although they have plans for a global reach.The U.S. patents are searchable in PDF format. Advanced Search provides most common field searching, including CCL and IPC.
Freepatentsonline has search fields similar to PATFT. It searches U.S., EP, (European), JP (Japanese), and WO (PCT) patents. Alerts, portfolios, and PDF downloading are available with free registration, but only for the sites' word-searchable patents (i.e., U.S. from 1976).
Patent Lens (http://www.patentlens.net/)
Patent Lens, created by CAMBIA, an independent, international non-profit, has a structured search and range of coverage similar to SurfIP. Country coverage is different, however: full text of PCT (1978-present), USPTO (AppFT, and PATFT (1976-present), EPO (1980-present) and IP Australia (applications and patents 1998-present).
The USPTO's PATFT searches and serves over 7,000,000 patents and includes all three types of patents (utility, design and plant). Word searches only find patents from 1976 to the present. Prior patents must be searched by issued date, patent number or classification number.
SciFinder Scholar: (University of Chicago Library subscription) Contains 4 million patents from over 54 countries. It's main coverage is chemistry and it is fairly comprehensive on this subject. It also provides indexing using technical (rather than legal) terms. Coverage varies based on topic. More information is here.
Derwent Innovations Index: This index is one of the most comprehensive databases of patent information. Over 11 million basic inventions and 20 million patents are covered from over 40 patent-issuing authorities dating back to 1963. Full text is provided for most patents.
QPAT-US : A subscription-based web service for patents and patent information that provides for an annual fee access to the Pluspat database, comprehensive patent family searching, detailed legal status information, unlimited PDF patent copies, and full-text searching of published applications and patents from the United States and the European Patent Office.