Google has published a set of lovely icons, designed for use in mobile apps or whatever else you fancy using them for.
The most intersting this is that they are for free.
This is part of its Material Design project.
The icons have just been published to Github, and there are over 750 of them neatly designed.
They’ll fit nicely in Android, but you could use the for anything, really. They’re available under a CC-BY-SACreative Commons license, so you can do whatever you want with them.