The Cloak of Elvenkind is a common magical item in almost any fantasy role-playing game (and computer RPGs as well). This cloak may also be called an Elven Cloak or an Elvish Cloak.
It appears that the first appearance of this item was in the novel The Fellowship of the Ring by J.R.R. Tolkien, where the members of the fellowship all received elven cloaks from a friendly elven monarch.
This plain grey cloak is designed to make the wearer nearly invisible. It works through an amazing chameleon-like effect, where it blends in to almost any surroundings.
The elves originally designed these cloaks so they could move invisibly through the forest (they provide almost perfect invisibility in a woodland setting). But, these cloaks have become more popular for their use in theivery, than for their forestry application.
These cloaks are almost always sized to fit a normal elf (most humans can wear them as well). Smaller ones to fit halflings and Dwarves are rare, but not entirely unknown.
This cloak can be rather easily constructed by an Elven mage and tailor working together. So far no other race has been able to manufacture an identical item (although the Cloak of Invisibilty is similar).
The band Marcy Playground wrote a song about this item entitled "A Cloak of Elvenkind", it appeared on their self titled album.