The QTR-1A reflectance sensor carries a single infrared LED and phototransistor pair in an inexpensive, tiny 0.5" x 0.3" module that can be mounted almost anywhere and is great for edge detection and line following applications. This sensor is sold in packs of two units.
XBee is good; Synapse is just better. More I/O, embedded python, and generally better range at comparable cost! This particular module is 2.4GHz with OTA (Over-The-Air) speeds up to 2Mbps, maximum 450M (1500') range and a chip antenna for compact installation.