A new service is on air as from December 2007: http://www.marinetraffic.com/ais/
Track vessels directions, find voyage-related information and see a map with current ship positions. The system is based on AIS (Automatic Identification System) which collects the current position on ships through VHF frequencies.
The covered range is currently limited at Aegean sea, east Mediterranean. The expansion of the range in other seas around the world depends on communinty efforts. This is an open project and we are inviting any volunteers wishing to install an antenna plus an AIS receiver and send the data they collect to the central server.
I also created a blog in greek language: http://marinetraffic-gr.blogspot.com