Lambda Diagrams originally appeared at [1].

[1] https://tromp.github.io/cl/diagrams.html

The submission page doesn't link to Tromp, but the YouTube videos do.

Tromp's page links to the submission:

> Paul Brauner has produced some awesome videos of beta reductions, produced with this software.

Also linked from Tromp, Cruz Godar's animated multi-colored calculator is my favorite:

https://cruzgodar.com/applets/lambda-calculus