Gamebryo

Numerical Design Limited first sold it as NetImmerse, and under that name it rendered The Elder Scrolls III: Morrowind in 2002. As Gamebryo, and after the merger that formed Emergent Game Technologies, it went on to power Civilization IV, Oblivion and Fallout 3. It was more toolkit than finished engine: a renderer, a scene graph and an animation system, with physics often bought separately from Havok.

That was both the strength and the weakness. Studios had to build a lot themselves, and Bethesda built so much that the result was eventually renamed the Creation Engine. The lineage shows in the file extension: models in Skyrim and Fallout 4 still ship as .nif — NetImmerse File.