The Apple TV has a lovely screensaver called "Aerial." It shows beautiful, drifting scenes from around the world (you can spot New York, San Francisco, London, Hawaii, China, and the list goes on). It's even synced up with the time of day, so if it's evening where you are, you see evening scenes...and so on.
Programmer John Coates has ported the screensaver to the Mac, and it's free. It downloads the same video files directly from Apple that the Apple TV screensaver does, so you'll see exactly what you get on Apple TV.
Installation is easy: visit this page and scroll down to the "Installation" section. The three steps are: download, unzip, and double-click the "Aerial.saver" file. Boom! (In my case, on that last one I had to control-click, and then click "Open" to get past security settings on my Mac.) So head on down to Screensaver Town.
Oh, and if you're a Windows user, guess what? Dmitry Sadakov has you covered. Thanks, internet!