Tailspin is an interactive notebook-style REPL in the browser. It combines an interface inspired by Mathematica and the IPython notebook with a "timeless" declarative programming model inspired by spreadsheets and, to a certain extent, Elm. It's sort of trying to become an authoring tool for explorable explanations.
A Tailspin document, or sheet, contains code cells that describe its interactive behavior. Code is written in a strict subset of the Clojure programming language that eschews impure functions and any notion of mutable state. Behind the scenes, data flows from cell to cell by means of a dependency graph. When the code in a cell is updated, the resulting change in that cell's value is automatically propagated downstream to any dependent cells.
Direct manipulation is one of Tailspin's defining features. A code cell's value may depend upon user input (taken from a variety of familiar UI elements) in addition to the code the cell contains. Through such input cells, the user may manipulate various properties of the underlying reactive system without having to edit any code.
Very WIP. Lots of conceptual stuff is still in flux and the current UI is more a proof of concept than anything else.
MIT License. Hack away.