wxWidgets - wxWidgets: Cross-Platform GUI Library
PixBuilder - Free Photo Editing Software for PC
Electron - Build cross platform desktop apps with web technologies
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.