Google Design have open-sourced 750 icons as part of the Material Design system icons pack. The system icons contain icons commonly used across different apps, such as icons used for media playback, communication, content editing, connectivity, and so on.
The Noun Project is a website that aggregates and catalogs symbols that are created and uploaded by graphic designers around the world, many available under Creative Commons licences.
Iconfinder provides beautiful icons to millions of designers, developers and other creative professionals. Iconfinder now hosts the worlds largest collection of premium icons, with over 340,000 icons in stock, many available under Creative Commons licences.