XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
digiKam - Professional Photo Management with the Power of Open Source
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
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.
Darktable - darktable is an open source photography workflow application and raw developer.