Built by Yotam Mann with friends on the Magenta and Creative Lab teams at Google. It uses TensorFlow, Tone.js and tools from the Magenta project. The open-source code is available here. To play, just click the keyboard, use your computer keys, or even plug in a MIDI keyboard.
This experiment lets you make music through machine learning. A neural network was trained on many MIDI examples and it learned about musical concepts, building a map of notes and timings. You just play a few notes, and see how the neural net responds.