Base UI Sketch Framework - Huge set of everything you need to design and prototype
Icon Slate - Icon Slate lets you easily compose, import and export desktop icons in many formats.
INSTAD.IO - Automagically converts vector images into sketchy style
Icon Generator - Icon Generator is a little application built on AIR that lets you generate a CS5 or Web 2.
Sketch Extensions - An official collection of Sketch extensions & plugins
IcoFX - IcoFX is an all-in-one solution for icon creation, extraction and editing.