A growing collection of simple and consistent icons specifically designed for systems and products. Use how you want, without attribution.

Icon fonts for web designers with added flexibility, including multiple button shapes and a unique multi-color option. Free and open source, Stackicons-Social includes finely-crafted icons for over 60 social brands. The Stackicons project is by Parker Bennett, a web designer and front-end developer based in Los Angeles.

On designing good icons.

700 free little icons.