XnGine was a 3D engine developed by Bethesda Softworks. In the 1990s support for higher resolutions was added and in 1998 it was made compatible with 3dfx graphics cards.
The Terminator: Future Shock was the first game to use the engine, and also the first 3D PC game to use the now popular mouse-look interface, which was initially unpopular with gamers.
Games which used the engine:
Originally, The Elder Scrolls III: Morrowind was supposed to use the engine but it was ultimately dropped in favor of NetImmerse (the predecessor to Gamebryo).