This video shows how three simple ingredients (soap, Hydrogen Peroxide, and dissolved Potassium Iodide) is all it takes to make a pretty neat, but relatively safe, chemical reaction. IMPORTANT - if you try this, make sure to wear goggles. As one of the commenters pointed out, hydrogen peroxide can burn your eyes.
Click play, or visit the http://www.metacafe.co.il/watch/706684/very_sudden_chemical_reaction/ [Metacafe], to see the instructions and results.