MrCrayfish's Key Mod
http://www.mrcrayfish.com/mods.php?id=ckm

uses minecraft's in built lock system to allow players to lock storage items preventing players without the key from accessing the block.

Would be useful for multiplayer environments

I second this, it would be a welcomed feature i would use often

Support for the ILockableContainer API is finally implemented for 1.8.9 and 1.9.4+. But since this support doesn't extend back to 1.8 and the Key mod appears to be discontinued in 1.8.9+, it's a moot point.