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!
SpeedTree - SpeedTree is a powerful toolkit used to create 3D vegetation for video games, films, and animations.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
The Grove 3D - Model or grow adaptive 3D Tree models for use in CG renderings.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
Tree Studio - Easy 2D Tree Creation
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.