Although you can compile the syntax of Python or Ruby into JavaScript fairly easily, the semantics don't match up very well.
You can run <script type="text/coffeescript"> today. All of the interactive bits on CoffeeScript.org are implemented with it -- and here's an example of a site that uses it for something a little more creative (View Source):
Not to mention that coffescript will work on any javascript runtime, not only browsers. It works on rhino, on node, for scripting Qt, and anywhere you can run js.
You can run <script type="text/coffeescript"> today. All of the interactive bits on CoffeeScript.org are implemented with it -- and here's an example of a site that uses it for something a little more creative (View Source):
http://thelincolnshirepoacher.com/