PhotoMiner lets you scan directories on your Mac to find all the photos residing in there.
Tom’s Editor works inside your browser and helps you view and edit your images on the web.
IrfanView allows you to view images of various formats and perform many tasks like optimization and conversion on them on your PC.
Memories is an open source image viewer that lets you import and organize your photo libraries on Linux.
vimage helps you view your images full-screen and without any ugly frames.
If you want to enable native viewing of HEIC images on your PC, CopyTrans HEIC will help you do that.
If you’re bored using the default image viewer, try LightGallery which is a modern app for viewing images in many great ways for your computer.