My son has taught Leo to come when he's called (at least most of the time, when he feels like it). It's so cool!
To call Leo, we quickly rub our palms together. Leo perks up and hops over. If we repeatedly call, wait for him to hop over, and move a short distance away, Leo follows. It's become our most reliable way to get Leo out of his favorite spot under the couch and back into the room where his X-pen is. Now, we're trying to always reinforce the behavior with a small reward - a piece of carrot, etc. On second thought, maybe he's trained us!
I asked my son what inspired the calling method. He said that a lot of cats come when you rub your fingers together, so he tried rubbing his hands. It probably doesn't hurt that my son usually serves Leo his morning greens. If I get the trick on video some day I'll post it.