wxWidgets - wxWidgets: Cross-Platform GUI Library
Creative Market - Buy and sell handcrafted, mousemade design content like vector patterns, icons, photoshop brushes, fonts and more at Creative Market.
Electron - Build cross platform desktop apps with web technologies
SocialSizes - Sketch templates for sharing images & videos on social media
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Designify - Turn any photo into awesome, automatically