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!
CopyTrans HEIC for Windows - A plugin that lets you view HEIC photos on Windows and convert them to JPG by right-clicking.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
iMazing HEIC Converter - iMazing HEIC converter is a free Mac and PC app which converts photos from HEIC to JPEG or PNG.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
Icaros - Icaros is a collection of lightweight, high quality, Windows Shell Extensions
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.