Step 1: Tools and Parts
We will program the Teensy to send a unique keystroke combination, Meta-Alt-Control-W, that will be intercepted on the computer - for Windows you can use AutoHotkey, for Mac you can use QuickSilver, and for Linux you can set it up directly in the Window Manager's preferences using xev or keytouch.
You'll need to do a little scripting, but the Lifehacker links I used above have many common script libraries to get you started.
Finally, you'll need an Easy Button.
For tools you'll need a P0 or P1 philips screwdriver, a soldering iron, and about a foot of hookup wire. A coping saw or hacksaw is helpful for cutting the hole for the usb port into the side of the button, but a pair of pliers will do just fine with the relatively soft plastic, it will just look a bit more ragged. Not pictured are an xacto knife or small flat screwdriver, and a wire cutter/stripper.
Disclaimer: I'm not affiliated with Microsoft, Apple, PJRC, Adafruit, Lifehacker, Quicksilver, Autohotkey, nor Staples. I just like their products and used them in this Instructable.
Any soldering or other hacking is at your own risk.
Step 2: Disassembling the Easy Button
Remove the four pads on the bottom of the button. Unscrew the four screws you'll find underneath the pads. Take off the top, which is in two parts.
There are two screws holding the board on, and two screws holding the mezzanine in place. Remove those. Under the metal click plate are two more screws. Remove those and remove the mezzanine. Save the two metal slugs which will fall out when you remove the mezzanine.
Take out the speaker, and clip the wires to the speaker and battery compartment.
Step 3: Modifying the Board
Solder one 6" wire to the ground plane.
Carefully scrape the solder mask off the line from the button. Scrape across the line near the epoxy dot to break that connection. Solder a 6" wire to the trace on the board - it may help to tape down the wire with electrical tape, I ended up pulling the copper right off the board several times.
Solder one wire to the GND pin on the Teensy, and the other to B4/pin 13.
Step 4: Programming the Teensy and Checking Your Work So Far
Open the Serial Monitor, you should see diagnostic messages when you press the button. If not, go back and check your connections. Make sure you've soldered to the right traces, and that you've cut the trace from the button.
You may get a popup asking you to identify a new keyboard, you can safely close this.
Step 5: Installing the Teensy Into the Button
Use wire to hold the Teensy down - I solder it to unused pins. Since the unused pins are inputs, there's no danger of short circuits by doing this, since the inputs are electrically disconnected.
Mark the points on the frame where the USB connector will go, then cut to right below the Staples logo. File the cut edges smooth for neatness and to avoid sharp points.
Step 6: Reassembling the Button
Screw in the four screws that hold in the mezzanine. Put the click plate back on, then screw in the board. Route the wires away from the four holes that the button top will go into.
Put the button top on, then the frame. Note that the frame has slots to line it up with the button top and screw holes.
Screw in the four bottom screws and replace the rubber pads.
Step 7: Setting Up the Hotkeys
Install the Quicksilver program and put the sample applescript (easy.scpt) in your Documents directory. Add the trigger and set the hotkey to Command-Alt-Control-W, as shown. I've also included a script that's specific to Mail.app that will tell you if you have new mail.
For Windows and Linux, set up your hotkey program as appropriate.
 I added an autohotkey.ahk file based on the example.
Plug in the easy button and press it. Your action should happen!
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