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!
https://gizmodo.com/what-material-design-means-for-the-future-of-android-1595976211
Just published to Github, there are 750 of the neatly designed icons. They’ll fit nicely in Android, but you could use the for anything, really. They’re available under a CC-BY-SA Creative Commons license, so you can do whatever you want with them. [Github via Kottke]