MicroG is an open source replacement for Google Play Services on devices running Android. Since Google Play Services form a large part of the backbone of Android (many apps use the services), it is a very helpful piece of software for people going NO GAPPS. It is developed almost solely by Marvin W, user MaR-V-iN on the XDA Forums. It is very much a work in progress, though it is plenty stable for everyday use and the core functionality is in place: location services, Google authentication, and Google Cloud Messaging, which many apps use for notifications. You can find the full implementation status on the project’s GitHub wiki.