A few interesting reactive notebooks off the top of my head:

- Pluto.jl (Julia)

- Clerk (Clojure)

- Polynote (Scala, Python?)

- Percival (Datalog + JavaScript)

As for Observable, note the runtime is open source. I don't know any perfectly polished alternatives that use it, but for now there's Starboard.gg (which if I'm not mistaken uses Observable's runtime, but someone else said it doesn't), and this VSCode extension: https://marketplace.visualstudio.com/items?itemName=GordonSm...

I'd love to see more alternatives in this space, specifically of reactive notebooks or similar environments.