PhotoQt is a simple, yet powerful and good looking image viewer, written in C++/Qt, published as open-source, and completely free. Developed by Lukas Spies
License: Open Source
FastStone Image Viewer is a feature-rich image viewer and editor with extensive functionality, making it suitable for detailed image work, but it's limited to Windows. In contrast, PhotoQt is a lightweight, cross-platform image viewer that focuses on speed and simplicity, making it ideal for users looking for a quick and efficient way to view images.