GIMP, the GNU Image Manipulation Program, is an excellent tool for editing images with many features comparable to Adobe Photoshop. It has been around for 25 years and is free and open source. Download: https://www.gimp.org/
Category: Open Source
Krita is a professional free and open source painting program. It is an excellent tool for concept art texture and matte painters illustrations and comics In addition, is supports 2D animation, and can be downloaded from Krita.org
Free and Open Source yet professionally powerful 2D animation software. Simulates a cell based animation approach, handles bit bitmap and vector layers.
Meshroom is a free, open-source 3D Reconstruction Software based on the AliceVision framework
This powerful photogrammertry tool is available for Windows and Linux