PhotoFlare - Quick, simple but powerful Cross Platform image editor.
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.
Photoline - PhotoLine: Bildbearbeitung, Layout, Vektoreditor, Bachkonverter und Webbildbearbeitung
PhoXo - PhoXo is small and free image editing application.
Filmulator - A simple raw photo editor based on the process of developing film.