htmx
htmx.org
A small JavaScript library that extends HTML with attributes for making AJAX requests, WebSocket connections, and CSS transitions directly in markup — enabling dynamic interfaces without writing JavaScript.
Why it is useful
Lets you add interactivity to server-rendered apps by returning HTML fragments from the server instead of JSON. The mental model is simpler than React or Vue for many use cases: no client-side state management, no build step, no hydration. Particularly powerful paired with a fast server-side framework like FastAPI or Rails. The documentation is unusually well-written and entertaining.