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.
So you have an Electric Imp (or plan on buying one), what's next? Well you're going to need a breakout for it, here's where the Electric Imp Shield comes in. This shield gives you to the interface to adapt an Arduino-compatible board to a WiFi enabled Cortex-M3 core development module.
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.