The Arduino is great, you can make it do pretty much anything you want.

I wanted a tripwire. This is just the circuit and the code, you can use it for quite a few things, like a trigger for a camera, or you could make it shut down your computer if someone crosses it.

When I have the time, I want to hook it up to an electric airsoft gun with a relay and have it shoot you when you break the beam. I suppose I'll add to the Instructable when that happens.

So let's start!

Step 1: Components Needed

This is what you need for this project:

-Arduino
-10k Resistor
-Photo resistor (Practically any value will work, the program will just need to be changed slightly)
-Laser Pointer
-Jumper Cables
-Alligator clips

Step 2: Circuit

Below is a picture of the circuit.

Connect one leg of the photoresistor to Analog Input 0, and the other leg to +5v. Then, take the 10K resistor and connect one leg to GND (ground) and the other to Analog Input 0.

Also, I wired an LED to Digital I/O pin 13 to show when the laser beam is broken.

Step 3: The Laser Pointer

To control the laser pointer, I did this:

1) Remove the end cap and take out the batteries.
2) Inside there should be a spring, connect one of the alligator clips to this spring.
3) My laser pointer (and all of the others I have seen) have a completed circuit when the end cap is on, so I connected the other alligator clip to the bit of metal on the inside of the laser pointer.
4) Connect one of the alligator clips to GND, and the other to pin 4 (or any pin, so that you can control the laser).
5) Lastly, use masking tape to tape down the button so the laser pointer is always on.

Step 4: The Program

Here is the program I used.
Pin 4 is the laser pointer, pin 13 is the LED, and the Analog Pin 0 is the photoresistor.

void setup() {
pinMode(4, OUTPUT);
pinMode(13, OUTPUT);
}

void loop(){
digitalWrite(4, HIGH);
if(analogRead(0) < 750){
digitalWrite(13, HIGH);
} else{
digitalWrite(13, LOW);
}
}

This program is highly specialized to the room I'm sitting in right now, you will have to calibrate it to work in the lighting conditions you have. To do this, you must read the value of the photoresistor when the laser is hitting it. Then, measure it when the laser is not hitting it.
To do that, you can use this program, and then monitor the Serial Output.

void setup() {
pinMode(4, OUTPUT);
Serial.begin(9600);
}

void loop(){
digitalWrite(4, HIGH);
Serial.println(analogRead(0));
}

When the laser beam hit the photoresistor I had a value of around 850. When I stopped the beam with my finger, I had a value of about 700. So, I made the LED turn on when the value drops below 750 in my program, indicating the beam has been broken.

Step 5: Test! (and Troubleshoot)

Run the program and see how it goes!
First off, you have to make sure that the laser pointer is on. If it is not, check the following:
-That the on button really is on.
-That you didn't connect the alligator clips the opposite way they should be. To fix this, change one from GND to Pin 4, and the other from Pin 4 to GND.

I hope this was mildly interesting!
I have a video of it in action below. Excuse the LED bargraph, I use it to display how many emails I have waiting for me and I didn't want to remove it...