I created my first ever Android app to download free piano sheet music in PDF format
This isn't necessarily a Nexus 5 app per se, but I do own the device and I love it.
I envision people using this app on their tablets to download sheet music and be able to play it right away.
If you don't own an android tablet, you can use it on your phone and I've added share functionality so that the PDFs can be sent right away from within the app, to say DropBox or other cloud file services so that they can be viewed on other devices or even printed.
If you guys play piano, let me know what you think, or maybe just spread the words to your friends that do.