PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
PhotoFlare - Quick, simple but powerful Cross Platform image editor.
PixBuilder - Free Photo Editing Software for PC
showFoto - ShowFoto is the standalone image editor of the digiKam project.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.
Photoline - PhotoLine: Bildbearbeitung, Layout, Vektoreditor, Bachkonverter und Webbildbearbeitung