AIS tracking become mandatory on all ships since 2004. The low price of AIS receiving equipment and simplicity of installation led to constantly increasing number of free AIS tracking web sites - amateur and professional ones. Here I will review only the top 3 of them: MarineTraffic, AISHub and ShipPlotter.
MarineTraffic uses Google Map technology for visualization of the ships. Currently (Sept.2010) the number of the ships in their site reaches 18-19K. MarineTraffic has been developed by the Aegean University.
AISHub is the only one free AIS sharing site which provides raw AIS NMEA sharing instead of visualization. It is precious resource for AIS professionals and software developers. Each member of AISHub shares his own AIS feed and receives access to all other feeds. Currently the number of the ships in AISHub server is between 6-7K.
Two years ago AISHub team released the best free AIS data forwarding utility - AISDispatcher. Until then the only possible solution for AIS data sharing was ShipPlotter. AISDispatcher is very light weight and the user can create complex structures of cascade connected instances of AISDispatcher. It runs from system tray and does not require a lot of system resources. AISDispatcher is also available for Linux.
AISHub also provides free version of the commercial AIS viewer VT Explorer
3) ShipPlotterShipPlotter was the first AIS sharing center. The tool is not completely free (it is $25) but provides unique abilities to decode AIS signals directly from scanner discriminator by using the sound card of your PC. ShipPlotter supports several chart formats and has the most advanced interface for AIS processing.