XnView is a super image viewer. This free software can read hundreds of image file formats, if you dare to name it one by one. It's fast, responsive and will make Windows Explorer thumbnail viewer looks way too obsolete. I love the way it displays all my humongous numbers of images instantly and with a built in file viewer, it can play video file formats too!
What really stands out for me in XnView is its responsiveness. It renders thumbnails for quick preview of your pics in record time and i really love all the bells and whistles that was included for quick image editing. And yeah, it's available for other platforms too! Linux users - enjoy yourself!
Download XnView for free at http://perso.wanadoo.fr/pierre.g/xnview/endownload.html
As an alternative, or if you love to compare free softwares, read my review on IrfanView, one of the most popular image viewer on earth.