Radio Shack had something a few years ago called the CueCat. It used a red LED to read the black barcode marks. You had to run the device over a barcode from side to side at a semi constant rate or it wouldn't read. I assume that it measured the space between and width of each bar and converted it into keyboard commands (it plugged into the keyboard PS2 port). Perhaps you could make an LED bounce light off the barcode and use a microcontroller to figure out the space between the bars. I'm sure you can find info on converting the spacing of standard barcodes online somewhere.
I guess the laser scanners work the same way but they are in constant motion and you dont have to 'scan' it accross the barcode.
~Mike
I had a bar-code scanner that worked with the HP 41 programmable Calculator. You could use it to program games and math equations.
There are many codes like 2 of 5 etc. There is a industry standard though too .
I also had at one point a light pen, that worked with the Commodore 64. You could use it to draw graphics on a computer screen. But that's not the same principle.