FastStone Image Viewer
A newer version of FastStone Image Viewer is available.Or, Learn More About FastStone Image Viewer
Full user review
"Outstanding photo viewer"
Lots of features in an intuitive package. Permits simple viewing and minor adjustment of a single image, or more or an explorer style interface with more than the usual tools for making photo quality edits, crops, rotations, etc. Much more than just a viewer. I have found no evidence of malware, found the program stable and haven't noticed any ads or solicitations.
At this price point (free): none.
For my purposes, it's the best viewer I've found; I've made it my default viewer.