The Call of the Fireflies is a puzzle-based singleplayer mod for Crysis. There's no guns, explosions or nanosuits involved. Just an old man who follows mysterious fireflies through mountains and dark caves, sleeping villages and frozen woods, and has to use all kinds of mechanisms to open his path, while battling to stay alive in the winter cold.
Interesting mod, the fact that it's purely a puzzle game actually is ideal for the cryengine 2, as at points the (mostly) slow pace allows you to pause and admire the beauty of the environment.
Good puzzles, innovative viewpoint and a quirky ending that makes you want to find out what happened makes this worth playing.