etcd is a data store that backs a sizable chunk of critical web infrastructure, like Vitess and Kubernetes. We also used it at Mixer in a system called Constellation, which I will one day write about. Several years ago they released their grpc-based "v3" API, which is more efficient and capable than the previous version.
In this client I maintain, to my knowledge, the only mature and stable Node.js client for their current API.
Using underlying etcd data store, etcd implements higher level constructs like software transactional memory (STM), elections, and soon hashrings. I also took the chance to play with TLA+ to prove correctness of the watcher implementation, which is one of the more intricate portions of the code.
Recently I've married etcd3 to Cockatiel for fault-handling.