Boss Alarm
AGlass0fMilk 
Resources and Going Further
Though it may seem like something out of a spy movie, you likely use this kind of technology every day. The infrared receiver in the owl's eye is the same you will find if you open up any modern TV or DVD player. That's right -- remote controls use this to send key presses!
To Infinity... and Beyond!
Infrared communication has applications beyond breakbeam sensors and remote controls. NASA is using infrared lasers to speed up data transmission to and from Mars! It is also used in modern fiber-optic communication systems. For more on infrared communication, read this page on Wikipedia.
Infrared is one of my favorite communication methods to use in my projects. It is a pretty lightweight, cheap, and versatile platform. All it takes is a common IR LED and an IR receiver diode.
Another project I made for a hackathon was a multi-user chatroom that transferred messages across the room using infrared! My team even transmitted a picture over the channel. We called it IRC (pun intended, InfraRed Chat).
You can use this design as a basis for a number of other projects! See if you can modify the circuits to trigger other things using an infrared breakbeam sensor.
Aside from those mentioned in the tutorial, here are a few resources I used along the way:
If you are having trouble with the Teensy, browse through the board's website to find more information and tutorials.
If you have any interest in microelectronics, you might like this article on the internals of the 555 timer!
For more project ideas, check out these other great SparkFun tutorials.