Hi-Def San Francisco
Hi-Def San Francisco have created an amazing Google Map that tracks in real time all the ships in San Francisco Bay. Every commercial vessel, and some large pleasure craft, in the bay transmit their position using AIS, which allows Hi-Def to up-date the position of ships on the map every 15 seconds.
Hi-Def also takes a photograph of every vessel as it crosses an imaginary line between the camera and the Trans America building. This means that when you click on a tagged ship on the map you can actually see a recent photograph of that very ship. As well as a recent picture, each vessel is tagged with its type, e.g tug, tanker etc, its current speed and its current heading.
If that isn't enough for you Hi-Def have also created an amazing Quick-Time video of a day's traffic tracked on a Google Map and this image of 30 day's cumulative traffic:
This image is cropped from the much larger original at Hi-Def San Francisco.
I could sit and watch this all day.
Other Ship Tracking
Hurtigruten - Scandinavian ship tracking