Here's one (okay, four): mirrors. Or windows. Or dark TV screens. Or shiny jewelry.
What do they have in common? Your character can see through them to an opposite world. A world where if she raises her eyebrow, the person in the mirror/window/screen/bracelet raises her left eyebrow at exactly the same time...or, less dramatically, your character can see reflections.
Reflections are an underused commodity. A hallway mirror can bounce an image from one room to the next, allowing a flash of what's going on inside--theft? murder? That Guy Your Character Secretly Loves hooking up with That Girl Everyone Knows is an Ice Queen?--to pass into the unsuspecting main character's visual cortex. Conversely, your character can channel a reflection in the direction he wants--maybe using the dark screen of a cell phone to watch the suspicious man/woman/baby/goblin behind him on the subway.
The point: mirrors aren't just for a character to peer into and arbitrarily describe every aspect of his/her appearance. They can be used for FUN.
(Another example like this is noises from another room--sound travels, and this can be a great way to introduce information. What was that crash? Or are the walls so thin the argument between Guy Your Character Secretly Loves and Ice Queen is audible two doors down? If so, lucky you--I mean, your character.)
What do you think are some unused plot commodities?