mmmmmmm..... pastries.........
I didn't want to come up with something that was already mentioned. . . first came to mind is teleportation, and then something along the same line as marichiko's idea, but I think I'm going to have to stick with that one.
I want to be able to alter people's moods. Make grumpy people become insanely happy for no good reason; to be able to put someone in their place; make someone feel insecure all of a sudden when they're getting uppity - calm someone down who's panicking about something they don't need to... oh, the fun I could have....
Whoohoo, I could be the happy drug!