Pygame is a set of Python modules designed for writing games. Pygame adds functionality on top of the excellent SDL library. This allows you to create fully featured games and multimedia programs in the python language. Pygame is highly portable and runs on nearly every platform and operating system. Pygame itself has been downloaded millions of times, and has had millions of visits to its website.
Pygame is free. Released under the GPL License, you can create open source, free, freeware, shareware, and commercial games with it. See the licence for full details.
For a nice introduction to pygame, examine the line-by-line chimp tutorial, and the introduction for python programmers or Chapters 17 to 20 of "Invent Your Own Computer Games with Python".