A set of magnets with UI widgets on them that you can use to prototype various apps on either a metal whiteboard or, you know, your fridge. An absolutely fantastic idea.
Fantastic article about the benefits of prototyping. It’s not easy to decide on what type of interaction design deliverable you should use (wireframes, pdd’s, prototypes, etc.), but this article makes a great case for prototypes.
A nice, and looks to be very complete wireframe stencil set from Graffletopia.