Repurposing "menu" button on Windows keyboards used in OS X

Windows keyboards map pretty intuitively to OS X, leaving only the "context menu" button a little purposeless. I would like the menu key to behave as another Command key.

How does one define what the "context menu" key on Windows keyboards does in OS X?

On this keyboard, the "context menu" key is directly under the "." (period/greater than) key; on the bottom row between "alt" and "ctrl" on the right side, two keys to the left of the "left arrow" key. What does it do in OS X, and what is it called?