Radix Icons - A crisp set of 15×15 icons. Open source.
Wannathis - 3d illustrations and freebies
MakeAppIcon - MakeAppIcon resizes and optimizes your icon designs into all formats needed for iOS and Android mobile app!
Craftwork - A collection of User Interface resources made by Craftwork
Bjango App Icon Templates - App icon templates for PS, Ai, Sketch and Affinity Designer
Icon Rewind - Get old app icons for your phone