A collection of over 18,000 free flat icons. Icons8 includes an icon for pretty much every scenario, available in any colour and several styles including iOS, Android, Windows and flat colour. All the icons are created by a single designer so your UI is guaranteed to look consistent, and can be downloaded as PNGs in several sizes for free or a several scalable formats for a fee. If there's something missing, you can request it and they'll make it for free. There is also a great OS X menu bar app that gives you quick access to the whole collection right from your desktop.