As part of its Material Design project, Google has published a set of lovely icons, designed for use in mobile apps or whatever else you fancy using them for. And they're free!
Just published to Github, there are 750 of the neatly designed icons. They will fit nicely in Android, but you could use the for anything, really. They're available under a CC-BY-SA Creative Commons licence, so you can do whatever you want with them. [Github via Kottke]