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.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
Seer - A Windows Quick Look Tool in Qt.
heictojpg.com - It is a HEIC to JPG converter.
Eye of Gnome (eog) - Simple image viewer for the GNOME desktop.