Icons8 - Free app for Mac & Windows already containing 39,800 icons. Allows to search and import icons…
Radix Icons - A crisp set of 15×15 icons. Open source.
Iconscout - Design Resource Marketplace.
Drawcon - Draw your own app icons
Craftwork - A collection of User Interface resources made by Craftwork
App Icon Template - Royalty free app icon creator for iOS, OS X and Android