XnView (Graphic Viewer)
I've been using this for ~8 years.. one of those programs which i'm ready to pay for, but it's free
doesn't matter what you do on your computer.. we all have photos and graphics on our drives, so i'd say this is a must have for everyone.
the only decent alternative is ACDSee, but it's not even close to XnView's simplicity and usability.
<!--QuoteEBegin-->Come to discover XnView, a software to view and convert graphic files, really simple to use! Support of more than 400 graphics formats! It's free, so do not hesitate!