Active Markdown is a combination of Tangle-like interactive documents, and a markup syntax for defining the interactions and relationships between variables.
I did not write this, nor have I seen the source yet. The author promises it will be on github soon.
I think that this is an exciting next step in seeing more tangle-style interactions.
Bret, do you have any opinions on this author's implementation? It is limited in scope vs your set of demos for Tangle, but the increased readability is appealing.