What are the most powerful apps that run 100% inside your browser (no backend)?

There’s something fascinating about tools that run entirely in the browser—no servers, no installs, no data leaving your machine.

I’m looking for apps that:

• Work fully offline or client-side

• Can help in day-to-day tasks

• Handle real workloads (not just demos)

A few I’ve found:

• excalidraw.com – collaborative whiteboard that works surprisingly well offline

• localonely.com – several in browser utilities related to image/PDF/document

• photopea.com – full PSD editing in the browser

What else deserves a spot here?