I am pretty excited right now about the german company Tinkerbots. I don’t think they are open themselves but they make use of open platforms and transport them in great ways.
VIDEO INTRO (2min)
VIDEO with Lego Use and Cool Tune (1min):
They are Arduino based and compatible. So they benefit from that platform and contribute to it.
They are Lego compatible – Lego Technic and normal Lego. They use Lego as an open platform (Legos protection is expired, it is basically an open standard/platform now) and it is good for them (more opportunities for their product) and good for Lego (more use-cases for Lego).
It is plastic though. But they basically added a whole new Layer to existing Lego. Therefore they “did not invent a new modular platform they hacked into an existing one” which is to some extend even sustainable.