Freefall Satellite Screen Saver is an app that tracks satellites. There are two files that come inside the DMG that you download from the developer's website. One is an .APP file, which you can run directly, and the other is a .SAVER file, which functions as a screensaver and is controlled by your screensaver settings.
When you first start the application, you see an image of planet Earth and tons of different satellites orbiting it. There are a lot of things that you can change. The first thing that I did was reducing the speed of the satellites. By default, they move a hundred times faster than they really do, which really made me nervous, so I went down to real-time speed. From the appearance tab, you can choose to show Earth Imagery or not, and you can choose from different types of Earth image (night time, day and night, topography, etc).
You can also add specified cities, political boundaries, coastlines and a grid. From the Satellites tab, you can choose which satellites you want the app to show. They are arranged into groups and you can uncheck those that you don't want to show. There is a search field that makes it easy to find a specific satellite. Another interesting thing that can be done is having the app show the satellites that go over a specific continent or city. You will be amazed when you see how many satellites are above you right now.