Ships, and many private yachts, carry the Automatic Identification System (AIS), which continuously transmits position data and static vessel information for the benefit of nearby ships, and in some cases also for shore-based traffic-control authorities.
MarineTraffic.org uses a worldwide network of volunteers to receive AIS transmissions from locations throughout the world and make this data available for display. You can look at a location or search for a specific vessel by name. AIS transmissions are fairly short-range, typically 15-60 miles dependent on antenna height, so there will be coverage gaps in the open ocean and in places where no volunteer receiver is nearby. Still, it looks like a significant % of the world’s coastlines and river mileage is covered.
Pretty cool!
FYI, They have an iPhone app.
Great tool for Shipspotting.
There are also apps that show the current and past locations of planes on scheduled airline flights.
Jonathan, re apps for aircraft information…the companies offering this service get their data from an FAA feed called ASDI (aircraft situation display in industry)…I believe there are similar feeds available in some other countries. The original data source in the US is the various FAA radar facilities around the country. (Not only airline flights are tracked, but any identifiable flight, for example, a general aviation flight on an Instrument flight plan. There is a provision which allows some operators to block out the data for their flights.
The maritime application works differently, in that the data being displayed by marinetraffic.org doesn’t come from a government source or a small number of government sources, but rather from a volunteer effort involving large numbers of individuals who have their own AIS receivers and pool the data via the Internet.
The aircraft apps use a variety of sources, one of which is ADS-B data broadcast by suitably equipped aircraft. Some of the receivers they use to collect the ADS-B data are part of ATC or airports, but some are private. See e.g. http://flightaware.com/adsb/
So more centralised than MarineTraffic.org
There are also avgeek groups that stream Air Traffic Control conversations e.g. http://www.liveatc.net/
A couple of years ago the freshly restored DH Mosquito KA114 was shipped from New Zealand to Virginia. Cue kiwi avgeeks becoming fervent ship-spotters, tracking the ship across the Pacific and Caribbean. It was also cool to see the custom container as the ship passed through the Panama Canal.