A Mann Co. Supply Crate Key is single-use tool item which is available at the Mann Co. Store. It was made tradable in the October 20, 2010 Patch. It is used for opening a Mann Co. Supply Crate, and can be applied to any series Crate in your backpack, where it is consumed, along with the crate, in the process.
To use the key, select the key then click the 'Use with...' button in the backpack and select the Crate you wish to open to start the process of applying it. When a crate is opened, a 5 second timer ticks down, alongside a sound effect of chains jangling and wood creaking. The player will receive one of a set of items, which are listed in the crate's description and dependent on its series. This item is announced to the server in a similar format to regular dropped items.