GeoTown is a GPS-based treasure hunt game. You get an image of a common place of interest, and the current distance to it, and then have to find it and answer a question about it. The system has both a Java-Server, hosted on Google AppEngine, and an Android-App. You can create Routes (collections of sights) on the website, and play those routes in the app.
Puut is an OpenSource alternative to the infamous Puush. Its only purpose is to upload screenshots. You have to host the server yourself, which gives the credit of all the data to you instead of puush. The Server is based around Node.js, which I love. It stores the images in a SQLite-Database, which is really bad practice, but it works.
Because Cryptocurrencies seem to be trending right now, I created my own fork of BitCoin, called PedaCoin (PEDA). It does not do anything interesting really. 1 PEDA is worth around 0€, which is not great but okay.
And many more...