If you have a broken pair of headphones, and an amplifier, then you can make a stompbox for free.
1.) Headphones (at least one of the speakers still needs to work)
Optional: 1/4" adapter (if your headphones don't already have a quarter inch output)
Step 1: Step 1: Plug in the Headphones
Plug the headphones into the INPUT of the amplifier. This is the jack that you would normally plug your guitar, bass, etc. into. It seems counterintuitive to plug headphones in here (like walking into a building through a door labelled "out"), but it totally works. Speakers and microphones are basically the same thing.
If the headphones have a 3.5mm or 1/8" output, then you may need a quarter inch adapter. You can purchase one of these adapters at most stores that carry audio-visual equipment, like Best Buy, Guitar Center, Radio Shack, etc.
Step 2: Step 2: STOMP AWAY!
When you step or tap on one of the headphones, it should produce a thumping sound from your amplifier. If you want to take that further, you can add other effects pedals to the signal chain. I'm especially impressed by the results of combining this stompbox with a reverb pedal. The echoes make it sound so much "bigger" than it does on its own.
Be careful of feedback, though. Turning the volume up too loud, or placing the headphones in front of your amplifier's speaker can cause some pretty harsh-sounding feedback squeals. Non-bueno.