Velocity Sensitive Drum Trigger From Old Headphones

With a little software and some stuff you have around the house, you can make a simple drum trigger. A fancier version would use a real drum pad, a piezo transducer, and a cord with a headphone-jack; but this can literally cost you nothing to play around with.
Using a free VST app you can use the mic in to control MIDI events. It can be used to control drum machines or simply trigger samples.

What you need:
set of old headphones (only one side needs to work)
mouse pad
empty soda can
cassette tape case (or whatever case you like)

Tools:
tape
glue
ruler
knife

I decided to make a 1.5 inch square pad for the trigger. I added a 1/16 inch gap for the hole, and cut a square 1 and 5/8 inches into the cassette case. I liked the case because the two spindles give the pad more support.
I also cut a notch on the side of the case for the cord to fit out of.

I cut the a 1.5 inch square piece from the mouse pad and the pop can.
I cut a second section of mouse pad slightly smaller than the lid of the cassette case.

Remove the plastic casing around the ear-bud, then cut a small hole in the larger section of mouse pad for the ear-bud to fit into. Make it smaller than the ear-bud so it can't slip through.
You can use a pair of broken headphones as you only need one speaker. Cut away the broken one.

Glue the large section of mouse pad to the inside of the case.
Glue the section of pop can to the under-side of the "trigger pad."
Fit the exposed ear-bud into the hole, and glue the "trigger pad" in place.
Make sure the metal of the pop can that is glued to the "trigger pad" is touching the metal of the ear-bud.

After this was all dry I used black tape to finish off the look and disguise the cassette case.
UNOPAD (get it?)

KTDrumTrigger
A plugin that triggers MIDI notes based on the sound level of the incoming audio stream in different frequency bands. It allows you to "detect" occurrences of percussive sounds in an audio stream and send out a MIDI event whenever that happens. Available as VST effect for MacOSX and Windows.

Virtual midi cable program ( MIDI Yoke or Maple Cable)

You can use Ableton, Cubase, Cakewalk, Fruity Loops, etc. to run the VST
OR
a Stand Alone VST Host. I suggest SAVIHost

-
Set the Midi Out of SAVIHost to Midi Yoke 1. Then in your audio app, set Midi In to Midi Yoke 1.

In the Wave Devices settings of SAVIHost set the Input port to your mic input.

In KTDrum, make sure you set the Threshold and Gain of the Trigger 1 high enough to trigger events, but not to low enough for ambient noise to set off.

Have Fun and make Great Music!