Pi is a new digital currency being developed by a group of Stanford PhDs. For a limited time, you can join the beta to earn Pi and help grow the network. To join Pi, follow this link https://minepi.com/CryptoScrimper and use my username CryptoScrimper as your invitation code. You will need the invitation code in order to sign up. You don't need to leave the app open for it to accumulate tokens, but you're supposed to 'check in' regularly.
I honestly don't know much about this but recently started using it in case it has value some day. They purposely do not want to list it on exchanges at first. They want it to be a more equitable distribution process rather than people with a lot of money being able to access it more than others--an approach I appreciate. And it doesn't use up your battery or anything so I figure it's worth trying out. It's obviously very early days for this project.