This is a slick display of shipping (taken from marine AIS information) in the English Channel (scroll down on the linked page). Boy, that’s a busy body of water.
The MarineTraffic page is a neat mashup with Google maps and marine AIS ship position information overlaid on it. You can see and follow shipping around various parts of the world. Not necessarily up-to-the minute (they state up to one hour delay) and certainly not for navigation, but interesting. Potentially useful to get a feel for shipping volumes and types of traffic for an area you’re planning to visit.