Ask HN: What are some companies building JS heavy "curation" tools on the web?

I am trying to identify companies that are building the most ambitious JS heavy "Curation" tools on the web.

Fwiw, "Curation" tools are by nature extremely good candidates as: 1) They Are not CRUD heavy. 2) They require great instant feedback loop and experience.

I have a few examples in my head, some obvious ones and some slightly off the beaten track, like:

1) Docs by Google - a curation tool for building documents.

2) "Editor" by Optimizely - a curation tool for changing 3rd party websites on the fly.

3) Balsamiq - a curation tool for creating wireframes.(Flash based, but still a great example).

4) Nitrous.io, Cloud9IDE - curation tools for writing code.

What are some other good examples?

The motivation for the question is to understand companies where the "JavaScript Engineer" role is at the core of the product.