Keys are items used to unlock gates. They are color coded for the gate they unlock, and are not tradeable.
The Blue Key is found in an eastern room of the Oakland Sewer
and unlocks a north gate in the same sewer
This key is found in the badlands bunker. It unlocks a gate in the Castle Sewer.
Some other items that don't resemble keys in appearance are used in the same or similar ways. Most of them are tradeable.
The Black Mask unlocks the ability to rebel against the King.
Boats act like keys when jumping off a dock into water. They are more than just keys, though, changing the appearance of the character in water, as well.
Like boats, boots allow players to walk on other terrain, though they don't change the character's appearance.
A Dead Player can be used to enter the badlands bunker if you don't have the required 60 PVP.
Fishing nets and rods, pickaxes, and the like unlock abilities to fish, mine, etc. at the correct spots.
Player Hats unlock the P2P teleport abilities.