XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Okular - Okular is a universal document viewer based developed by KDE.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.