Coder from Google Creative Lab looks really interesting:

Now I’ve got a Raspberry Pi, and I will set this up for my kids to try out, but I wanted to try it out now (and downloading ~1GB on my rural ADSL connection takes time).

Coder itself is Node.js, so you can try it directly on you computer. These instructions are for Ubuntu (naturally), but you should be able to adapt them.

First make sure you have node installed:

$ sudo add-apt-repository ppa:chris-lea/node.js
$ sudo apt-get update
$ sudo apt-get install nodejs

Next create a pi user (Coder looks for this):

$ sudo sudo adduser pi

Now check out the Coder repository:

$ git clone https://github.com/googlecreativelab/coder.git

Install the dependencies:

$ cd coder/coder-base
$ npm install

…and run it:

$ sudo npm start

(You need to run with sudo because it changes the password for the pi user.)

Now visit https://localhost:8081 (it only runs over SSL), accept the self-signed certification, and play away!