3D Model Viewer is a free utility app developed by Defiant Technologies LLC that allows Android users to view 3D models of different formats. The app supports STL, OBJ, and PLY models, and it can become the default handler for opening these types of files from any other app, including your browser or file manager.

The app's interface is user-friendly, and it allows you to view 3D models that you have downloaded or open them directly from your browser. You can tap and drag to rotate the model, or pinch to zoom. Additionally, the app has a VR mode that you can switch to by tapping the VR button at the bottom of the display. This feature allows you to view the model in your favorite VR headset, such as Cardboard or Daydream.

Overall, 3D Model Viewer is a handy tool for anyone who needs to view 3D models on their Android device. Its support for different formats and the ability to become the default handler make it a must-have app for anyone who works with 3D models.