Jason Kottke links to a thrilling (yes, thrilling) animated video showing the evolution of global shipping routes between the mid-18th century and mid-19th century:

Here’s a fuller explanation from Ben Schmidt, who made the video: “This shows mostly Spanish, Dutch, and English routes — they are surprisingly constant over the period (although some empires drop in and out of the record), but the individual voyages are fun. And there are some macro patterns--the move of British trade towards India, the effect of the American Revolution and the Napoleonic Wars, and so on.”

Also note the cod and whaling fleets that appear and disappear with the seasons.