Cedar

Cedar

Unlocked by completing Cedar's Challenge

Jungle Cedar

Unlocked by completing Jungle Cedar's Challenge

Tribal Cedar

Unlocked by completing Tribal Cedar's Bonus

Chippendale Cedar

Unlocked by winning a multiplayer game

Gladiator Cedar

Unlocked by obtaining the "Marathon" achievement (removed)

Hippie Cedar

Unlocked by obtaining the "Don't tell me what to do" achievement

Dragon Cedar

Unlocked by resetting over 9000 times

Skins written in red are not unlockable in-game.
pictures taken from Tom Rutjens' website & in-game