XnView MP stands for Multi Platform = it is available for Windows, Linux and MacOS. It is a continuation of the older XnView Classic. In my opinion it is a great alternative to Windows photos app or any Linux image viewer that comes preinstalled. I even think it is an alternative to IrfanView. It is really fast and simple to use, supports more than 500 image formats, has built in folder browser, image converter, editor... AND MORE!
IrfanView - IrfanView ... one of the most popular viewers worldwide.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.
Axure RP - The prototyping and specification software used by 87% of the Fortune 100. Beyond mockups, design interactive prototypes for business solutions, websites, and applications.