Folsom Labs, San Francisco, Full Time Software Engineers (www.folsomlabs.com)
We make powerful design tools for Solar PV (think cloud-based AutoCAD with built in physics modeling). Our mission is to make these tools as easy to use as possible, which dramatically broadens the availability of solar.  It's a unique place in the solar industry, new technologies desperately need software like ours to convince both banks and installers that they can have positive impact on energy production.
We launched to revenue in January, and have real traction and high growth, and we're still just a 4 person team, so there's a ton of opportunity.   Everyday we get to deal with a range of problems that few startups get to offer – we have a modern web-stack, we need to use our cloud-clusters for the simulation half of the product, but also get to solve interesting physics/optimization problems on a regular basis (if that's your thing).
– Paul REDACTED_EMAIL
[1] <a href="http://www.greentechmedia.com/articles/read/better-faster-ba.." >.
[2] AngularJS, Python/Flask (API/Backend), Cython/C (Physics Simulation Engine)