1 Sharp GP2D120 IR Sensor - 11.5" / Analogue
11.5" or another range will do. Only do not buy the "ÃÂDigital version" of the Sharp sensors for this kind of project, they do not measure distance as the analogue ones does.
Price: 10 USDGet it here
Be sure to get the red/black/white wires for it. This is not allways included, and it is a non-standard socket!
This is actually not a favorite of mine, I usually use ultrasonic sensors, such as the SRF05 (find it anywhere via Google - they also sell it at the picaxe-store
picaxe-store where they call it SRF005 and have a picture of the back of an SRF04 in the shop! But it is the right one, and I did tell them but..). Anyway; The SRF05 is much more reliable and precise. It is also faster, but costs a little more, is a little more complicated to write code to, and a little more complex to install - so it is not used here, but if you are fresh, buy one of these instead ;)
If you go for the SRF05, I have made a small walkthrough to connecting the SRF05 here on letsmakerobots.com