Mikrokosmos
Mikrokosmos is an untyped and simply typed λ-calculus interpreter, borrowing its name from the series of progressive piano études Mikrokosmos written by Bela Bartok. It aims to provide students with a tool to learn and understand the λ-calculus.
Since version 0.3.0, it comes bundled with a Jupyter Notebook kernel, a tutorial on lambda calculus and specific modes for SKI combinators. Thanks to GHCJS and Codemirror, you can also try the Mikrokosmos lambda interpreter in your browser. Press the evaluate button there and play with the code!