Unity3d File Viewer Today
No 3D preview. You need to export to FBX/OBJ to view models externally. 3. DevX Unity Asset Viewer (Online / Web-Based) Platform: Any browser Cost: Freemium
For quick, no-install viewing, DevX offers a web-based Unity asset viewer. You upload a .unity3d or asset bundle file, and it renders the 3D model in your browser using WebGL. unity3d file viewer
If you have the actual Unity project source files (not a compiled game), the Unity Editor itself is the best viewer. You can open .unity scene files, inspect GameObjects, view shaders, and run the game in Play Mode. No 3D preview