Nomacs is a free, open source image viewer, which supports multiple platforms. Developed by Markus Diem, Stefan Fiel and Florian Kleber
License: Open Source
ACDSee is a feature-rich, paid application suitable for users needing advanced photo editing and organization tools, while Nomacs is a lightweight, open-source alternative that excels in speed and customization but lacks some of the advanced features of ACDSee.