You get these after getting a certain amount of skill points or doing other tasks.

Unlockable - How to Unlock

Clank Backpacks - Get 100 skill points
Concept Art Pack 1 - Get 10 skill points
Concept Art Pack 2 - Get 20 skill points
Flip Levels - Get 70 skill points
Movie Player - Beat the game
MP Mechanic Skin - Get 126 skill points
MP Soldier Skin - Beat the game on superhuman
The Mighty Wrench (allies get a wrench) - Get 40 skill points