A natural monopoly would be the Stock Market. If another one was to emurge, it would cause too much confusion to make it worth it.
A geographic monopoly would be the Jersey Shore. There is only one Jersey Shore and there is no way another one could be made.
A technological monopoly would be drug that cured any type of cancer. It would most likely be very hard to copy and would only be made by one company.
A government monopoly would be the NJ Turnpike. It is controled by the government, there is only one, and it would not make sense to have another one.