I recently checked out a book from the local library on Python (Learning Python, 2nd Ed.) and don’t think it is a very good language. It is too loose where it shouldn’t be, and restrains you where it shouldn’t.
JS is cross platform. Odds are that the browser you are using can interpret JS. Excellent quality web applications are written in it (Flickr, Google Maps, and GMail come to mind). And if you want to do something more ambitious than a web app (perhaps an app that can save files, for example) you can make a piece of Mozilla chrome (XUL Planet has an excellent tutorial) that will run on any computer that is running any Mozilla.org application.