A Unity3D file viewer typically refers to one of three things: a way to open legacy .unity3d web player files, a tool to inspect and extract assets from a built Unity game, or a custom-built utility within Unity to view files at runtime. 1. Opening Legacy .unity3d Files
A Unity3D file viewer must deserialize (unpack) these binary blobs. Unlike a PDF or a JPEG, you cannot just double-click a Unity asset; you need a tool that understands Unity's serialization engine. unity3d file viewer
A file viewer is useless if you can't look around. You need a script to orbit the camera around the loaded model. A Unity3D file viewer typically refers to one
Verdict: If you are on Windows and need a reliable, visual way to inspect a build, this is your first stop. How it works: You create an Editor script
GLTF (GL Transmission Format) is widely considered the "JPEG of 3D." It is the most robust choice for a Unity file viewer.
Noesis is famous for viewing proprietary game models. While not a full Unity viewer, it supports Unity's Mesh and SkinnedMeshRenderer formats via plugins.