Companies like exactEarth and ORBCOMM are using the Automatic Identification System (AIS) signals to track ships from space. A lot of the resulting information on maritime traffic has never been available before, particularly in real-time. The graphical displays of the traffic are quite interesting: Ahoy! Your ship is being tracked from orbit – BBC.
Here’s a promotional video from exactEarth that discusses the types of AIS satellite data and how they can be used: