n2yo.com is a mashup site that combines Google and Yahoo maps with entities in the sky. They track many satellites including Navstar/GPS, Iridium, and the ISS. So if your out on the ocean and see something bright go over, this site can probably help you figure out what it was.
We have a little time left before hurricane season in the Americas, but they also have a graphical real time hurricane tracker.
Thanks to Matt for suggesting this site.