The recent stories regarding the Somali pirates piqued my interest in the issue of ship tracking. I did a bit of this back in the day and always thought it was something that would lend itself to a web-based application, so I went looking for it. Lo and behold, there they were -- in droves!
The Automatic Identification System (AIS to the naval types) is used to locate and identify ships. It is in place on a wide variety of ships, including virtually all cargo and passenger ships, and there are a number of sites on the web that track, in more or less real time, the world's movement of cargo and people over the water. It does not appear to be used by military vessels but I would appreciate some insight in the comments from any reader who spent more time on the water than I.
While the main Wikipedia article on AIS lists a number of regional free sites, like shipais.com, and there are some commercial services such as AIS Live and Ship Plotter, I was most impressed by those sites that at least attempted to cover the multiple regions of the world for free. The first, sailwx.info, works by clicking on the map to zoom in and zoom out. Data on the map is limited to general location and sometimes the name of the ship.
Even better is VesselTracker, which covers a large number of mostly European ports and contains better ship data (see screenshot below). VesselTracker also supports an apparently large group of people who like to take pictures of ships, so there is a good bit of additional information associated with the site as well.
My favorite, though, is MarineTraffic.com (sponsored by the University of the Aegean) which provides a live feed of many ships in ports around the world. The amount of data here is phenomenal and the pop-up shots of ships at sea often include details about the ship and a thumbnail picture of it. See the screen shot below but click on it to go to the site.
Finally, if you are more interested in what is inside those ships than where they are, the best source of this data is PIERS. It is a commercial service but I have had several chances to see their product in action and, if you need this kind of data, it is a one stop shop kind of place to get it.