GPSActionReplay is a cool Java applet that lets you plot multiple GPS tracks over a map or image. It animates the tracks so you can see how each track formed. The obvious application of this is to replay races. The app has a bunch of extra features for sail racing like wind charts, etc. The UI is a bit confusing, but it's a fun way to watch and actually pretty educational (well, to sailors at least).

Here's the replay from a few boats in the race I did this week (our track isn't in there yet).