We've been using it to render schematic symbols and PCB footprints at CircuitHub.com (this has been going on for a couple of months). I'm testing it out in other areas of the product at the moment, so if all goes well additional Elm widgets might land on our site very soon.

Dreamwriter uses it, although Dreamwriter is still pre-1.0.

https://github.com/rtfeldman/dreamwriter

Here on PlayTacks, a sailing simulation: http://www.playtacks.com/

Nothing serious though...