How Do You Clean Crayon Marks Off Stuff?

My little two year old Picasso or Michelangelo got creative and decided to tag certain things around the house with his scribble. Problem is, it's on different surfaces.

We're talking on the window, on a leather couch, walls, everywhere. I know some of you parents reading this are having flashbacks saying "been there, cleaned that."

Help me out! What's the best way to clean crayon off? Thanks!