FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
CopyTrans HEIC for Windows - A plugin that lets you view HEIC photos on Windows and convert them to JPG by right-clicking.
Phiewer - Image Viewer - Phiewer is the easiest and fastest way to view your images, videos and even audio files.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
iMazing HEIC Converter - iMazing HEIC converter is a free Mac and PC app which converts photos from HEIC to JPEG or PNG.
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.