The QTR-1RC reflectance sensor carries a single infrared LED and phototransistor pair in an inexpensive, tiny 12.7mm x 7.6mm module that can be mounted almost anywhere and is great for edge detection and line following applications. The output is designed to be measured by a digital I/O line. This sensor is sold in packs of two units.
More info: http://www.pololu.com/product/2459