Turns out you have various choices to proceed.
2) Use Turbulenz which went open source this week. From my experiments today it’s a game engine done right – it has a building tools (written in python – yay!), it has tons of features and (gasp!) loads of documentation. The thing that bought me was the fact that I had no problems to install and run it and especially the impressive performance of this Quake 4 asset import:
4) If you just want to start without any installation – the Pixieengine is an online IDE (editor) and you can develop right away from inside your browser (and export it or embed an iframe if the environment allows it).
So get out there and roll the bones!