For mobile file managers, if you don't like the old UI of the classic Root Explorer or the bad usability of ES File Explorer, you can give Solid Explorer a try. Compared to similar products, its user interface is more modern and it's more feature-rich than its peers. If you feel that its features are not enough or its UI is not what you want, you can also extend it with plug-ins and icons pack.

Open 3D Model Viewer (AKA: open3mod) is a free and open-source universal 3D model file viewing tool, specially made for 3D designers, 3D printing enthusiasts and game programmers. It is developed based on the portable, open-source and cross-platform 3D model import library - Assimp (short for Open Asset Import Library). That's why it's able to import various well-known 3D model formats in a uniform manner.