Go to "Setup-bp" in your main S.T.A.L.K.E.R. dir. Chose your language and for Password # 1 input "pseudodog" to unlock the multiplayer map Pool as well as the UK and US military multiplayer skins. For Password # 2 input "snork" to unlock the multiplayer map Dark Valley as well as the French and German military multiplayer skins. These two codes are exclusive to the Limited Edition of S.T.A.L.K.E.R.

Password - Effect

snork - Multiplayer map Dark Valley, French and German military multiplayer skins.
pseudodog - Multiplayer map Pool, UK and US military multiplayer skins.