In online multi-player mode go to "Soldier Loadout", press start, then enter the corresponding password to unlock the following items:

ITEM - PASSWORD
Cloven Bandana - 8cba 3462 6595 14d4
Cloven Helmet - f6e0 fa3e f1f4 61e1
Cloven Helmet w/ Bandana - a7f6 6e7c 05ec 1728
Cloven Bearded Head - d708 6b08 0f1c b5f6
Cloven Skin - a8f4 d189 68f8 559e

Hybrid skin cooling apparatus:
When in the Hybrid skin you can turn off your cooling apparatus by pressing L2. This allows you to sprint and see through walls.

Human skin sprint:
While running in the Human skin you can sprint faster by tapping L2.