PhotoFlare - Quick, simple but powerful Cross Platform image editor.
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
Pixel Art Studio - A purpose-built drawing tool for pixel artists
showFoto - ShowFoto is the standalone image editor of the digiKam project.
Pixilart - A drawing app for delightfully retro pixel art