I'm not sure what "powerful tools ... in the browser" means for you, but here are some pure front-end JavaScript projects that I find to be powerful:

  1. "Live" documents/notebooks like:
  * Tangle - http://worrydream.com/Tangle/
  * Mozilla Iodide - https://hacks.mozilla.org/2019/03/iodide-an-experimental-tool-for-scientific-communicatiodide-for-scientific-communication-exploration-on-the-web/

  2. Visualization libraries:
  * D3.js - https://d3js.org/
  * Vega - https://vega.github.io/vega/

  3. Processing - http://processingjs.org/