There are a bazillion image viewers out there. And even though I practically live in Adobe Creative Cloud, I still need a light, fast viewer for exploring photos.
Faststoneimage viewer is the best completely free image viewer/toolbox I’ve found. I can perform conversions, crops, and straightforward image adjustments without having to enter Photoshop. Creating contact sheets is blindingly fast.
The UI is a bit dense, but it is feature rich. This is an excellent complement to any graphic toolset even if it seems a bit dated at first.