MobX 6
MobX is possibly my favorite open-source project. The elegance with which it makes an entire problem-space just <i>disappear</i>, with minimal magic or surprises, with maximal performance, with a delightful user-experience, with minimal opinions or assumptions about how you use it. I have nothing but praise; React + MobX is the most productive I've ever been while working on user interfaces.<p>On top of all that, Michael is a very reasonable and patient project leader. His documentation puts advanced concepts in plain terms, and when people file feature-requests he always displays a genuine willingness to consider their use-cases and points of view, to solve their problem if it's reasonable for the project to do so, and is never condescending even when questions or suggestions are plainly bad.