Photoscape - high quality photo editing software, that enables you to fix and enhance photos.

PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.

Pixelmator - Pixelmator is an image-editing application for Mac and iPad.

PhotoFlare - Quick, simple but powerful Cross Platform image editor.

ACDSee - ACDSee is an image viewer that uses very little system resources, and it has a small, sleek size that won't take up much space on your system.

Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.