PureRef - The simple way to view and organize your multiple reference images.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
nomacs - nomacs is a free, open source image viewer, which supports mu
IrfanView - IrfanView ... one of the most popular viewers worldwide.
DroidRef - Android image reference board app for artists
opti - simple, transparent media viewer