I built https://www.liquidityapp.com/ over the summer of 2015: "Liquidity is a smartphone based currency built for Monopoly and all board and tabletop games.", and I actively maintain it. I have enhancement ideas too, but I barely have any time to put into it these days so it is primarily just maintenance.

It's one part Android client and one part Scala backend (though there's some Scala in the client too). While I'm fairly pleased with the UX and UI I was able to create (given that I really don't consider them my speciality), the backend is the bit that keeps my interest now. That uses Akka, Akka Persistence (i.e. it's event sourced), and Akka HTTP among other things. The clients communicate with it via websockets.

The details aren't exposed to users but it uses public key authentication so as to not burden users with passwords/PINs. Each app generates a keypair when first used, and QR codes are used to make changing account ownership simple.