The meetups project is a sample application built in Java using the Play Framework backed by MongoDB. The app is meant to allow people to post and find upcoming meetups (or events, if you prefer).
The idea is to help people learn or familiarize themselves with MongoDB.
Accompanying the application the complete Guide which takes a step by step approach to building the application. You can dive into the finished application, residing on the master branch, or download a tag and build alongside the guide.