The OpenSky Network is a community-based receiver network which continuously collects air traffic surveillance data. Unlike other systems, OpenSky keeps the collected data forever and makes it available to researchers from different fields. With almost 50 billion ADS-B messages collected so far, the OpenSky Network exhibits the largest air traffic surveillance dataset of its kind in the world. In this talk, we explain how the OpenSky Network functions, what you can do with our data and how you can participate. We will show how you can easily build an ADS-B base station under 100 Euro. Composed from the cheap off-the-shelf components, this station will be still powerful enough to "see" planes in the radius of up to 150km.