HT: A Burgess
This site shows you all the ships currently tracked by this site and married to Google maps. Fascinating. Check out the English channel or the panama canal. Shows passenger vessels, cargo vessels, tankers, etc. etc.
I went to look at the northern and western Indian Ocean but couldn't see any ships. Obviously the database isn't fully updated, but still, cool :)
I know, I know, I am a nerd. This is like a giant railway set :)