Ok, so maybe Vietcry (opens in new tab) doesn't deviate that steeply from Crysis—they're both about shooting bewildered soldiers in the middle of a jungle—but the Vietnam War is a natural setting for the lush canopies and open maps of the FPS. Skirting the war's more dramatic tussles with morality and politics enshrined in classic films such as Apocalypse Now, the German-made Vietcry hands you the guns but yanks the pivotal Nanosuit and its maximum overpowered-ness.
The mod shows remarkable polish after a lengthy 10 months in boot camp. Though stripping Crysis' sci-fi connections is a nod toward keeping things believable—well, as believable as blowing up a car with just a few pistol shots allows—the changes also bring in old-school FPS mechanics: med-kits and armor jackets replace regenerating health, so you'll need to scrounge for protection and ammo during the mission.
Grab Vietcry from Mod DB (opens in new tab) . There's also a multi-cultural trailer for viewing, featuring German-speaking American soldiers trading lead with Vietnamese fighters.