XnView
Used by 2 peoplexnview.com ↗
“For fast browsing PSDs, XnView is great, especially for scoring screenshots to be used for marketing.”
Theresa Duringer Interview, Game developer, designer (Temple Gates Games) · via Uses This 
“I use an adorable and free image viewing/editing/organizing tool called XnView, which is an awesome small and fast swiss army knife.”
Derek Chatwood Interview, Game designer, illustrator · via Uses This