AIS, or Automatic Identification System is not much unlike APRS - it is a professional tracking system for ships. It runs at 9600 bit/s (GMSK) on two marine VHF frequencies, with a nice binary protocol which features automatic synchronization and time multiplexing - even with a very high number of vessels transmitting position reports once every 2 to 10 seconds, packet collisions will not be a problem. The protocol is standardized and well-defined, but unfortunately patented.
Fortunately the protocol is unencrypted and available. There is a Windows application called ShipPlotter which can decode AIS traffic and export the received position reports using HTTP. If you live somewhere near a coast, set up a 160 MHz FM receiver with a discriminator output or data port (you can't properly decode 9600 bit/s from the audio output of most FM receivers), plug it in the sound card of your PC, and you can track ships from miles away. A single receiver on the south coast of Finland can hear ships from Tallinn and a good part of the Gulf of Finland.
Now, aprs.he.fi can receive the HTTP position report exports sent by ShipPlotter. If you run it somewhere on a 24/7 basis, please get in touch with me and share the view with others! There's currently one receiver feeding us with AIS traffic from Helsinki, Gulf of Finland (thanks Tapio!).
To see some AIS traffic, zoom to the area between Helsinki, Finland and Tallinn, Estonia and make sure you have enabled viewing AIS data in the Options!