Firebase

Building and supporting a modern mobile or web application can be very difficult, especially if your development team is small. This is where Firebase comes in. It offers a whole range of important functionality right out of the box, such authentication services that have built in support for Facebook, Google, and Twitter. Other terrific services include file storage for your app and database access, as well as crash reporting and notifications. Best of all, all of these services start out free, and the free level offers generous usage allowances. You will only start paying for the services when your app becomes heavily used and you are already hopefully earning money from it. Even after this happens, their services are affordable. On the down side, their querying and indexing capabilities are limited and often require manual effort. There is also no support for SQL databases. It further has not really been battle tested yet with big and popular apps, and finally it really locks you into the product. Once you decide to use it, it will not be easy to do it a different way or to do it with a different company in the future.