PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
PhotoFlare - Quick, simple but powerful Cross Platform image editor.
Invision - Prototyping and collaboration for design teams
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.
Sketch - Professional digital design for Mac.