This is a sortable list of first-person shooter engines.
List of first-person shooter engines Wikipedia
Some features may or may not be integrated into engines. For instance for trees and foliage a special "engine" is available, SpeedTree, that does just that (or could be integrated into general engines). The Euphoria character's 3D animating engine can be used independently but is also integrated in the Rockstar Advanced Game Engine and the game Grand Theft Auto IV.