Another in the canon of Things I Never Got Round To Finishing.
In 2010 I had the brilliant idea of formalising all the handwaving explanations of Scottish Country Dancing into an XML format that describes exactly who does what when, and writing a web app that could animate any reel that you fed into it.
Here's a video of where I got to with it. It actually looks a lot better than I'd remembered, but now (2017) I don't think I even have the source code any more. Ah well.