This guy visualizes the microwaves from a microwave oven using a grid of neon bulbs and explains their distribution pattern. Brilliant!

Here's his explanation for the experiment:

Microwaves are invisible, so you can't see them inside microwave oven, but their presence can be detected with neon lamps. The changing electromagnetic field from the microwaves will make charged particles move, and so the electrons in the metal legs will move creating current. This current makes the lamps glow.