1). http://www.uspto.gov/patft/ (Us Patent Server) You'll need the plug-in which I have here.
2). http://www.delphion.com (IBM Patent Server)
3.) http://www.inpi.gov.br/pesq_patentes/patentes.htm (Brazilian patent server)
4). http://www1.ipdl.jpo.go.jp/PA1/cgi-bin/PA1INIT?989958673100 (Japan Patent Server)
5.) http://www.depatisnet.de (German Patent Server)
6.) http://ep.espacenet.com/ (European Patent Server)
7.) http://patents1.ic.gc.ca/srch_adv-e.html (Canadian Patent Server)
NOTE: For NON- English pages bring up a second window from here, Copy the information into the translation window to read it.
A utility patent may be expired just because the maintenance fees were not paid, as outlined below.
All utility patents which issue from applications filed on and after December 12, 1980 are subject to the payment of maintenance fees which must be paid to maintain the patent in force. These fees are due at 3 ½, 7 ½ and 11 ½ years from the date the patent is granted and can be paid without a surcharge during the "window-period" which is the six month period preceding each due date, e.g., 3 years to 3 years and six months. (See fee schedule for a list of maintenance fees.)
Failure to pay the current maintenance fee on time may result in expiration of the patent. A 6-month grace period is provided when the maintenance fee may be paid with a surcharge. The grace period is the 6-month period immediately following the due date. The Patent and Trademark Office does not mail notices to patent owners that maintenance fees are due. If, however, the maintenance fee is not paid on time, efforts are made to remind the responsible party that the maintenance fee may be paid during the grace period with a surcharge.
Note: The US military reviews all patents before a examiner sees them. The Government could take a patent away from you and classify it as secret. [Title 35, chapter 17, part 181]
Another reason not to patent, copycat patents are now legal;