Try Python -- a Python statement evaluator by Mike Meyer. Basically it tries to emulate a typical Python console inside your web browser using DHTML. The actual evaluation is done on the server side using remote scripting (aka AJAX). It lets you run Python with nothing but your web browser.
Server side runs Python in some kind of sand box, and quite a few modules are missing for security reasons. It just needs to have a nicer presentation (like Try Ruby).
And please, please give us console emulation of IPython inside a web browser, with all the auto-completion goodies :)