XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
PixBuilder - Free Photo Editing Software for PC
IrfanView - IrfanView ... one of the most popular viewers worldwide.
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.