What are the most interesting open-source projects in artificial intelligence and machine learning?

Some work on semantic transfer is quite interesting. https://nucl.ai/blog/neural-doodles/

(The code there is written by an ex-gamedev, too.)

I don't know much about the topic, but to get any useful answers, you should probably more clearly define what "most interesting" means.

That is, quite likely, we do not agree on the meaning of "most interesting", so any answer I'd give is likely not what you are looking for.

ML is a broad field, I mean it. In a single search you can find NN related stuff (with almost no effort), specially image processing. UDACITY has also crapton of material if you want to learn... years worth of learning material. RL seems to be somehow forgotten in practice: I heard of burlap, but I'm not programming with java at the moment so I can only point to it.