Distance sensors

Distance sensors are relatively expensive.

Here is a list of some of the cheapest:

Infrared:

small distances:

http://home.cogeco.ca/~rpaisley4/IrProximity.html

intermediate:

SHARP GP2D12: (10-80 cm) examples and connections:

http://www.acroname.com/robotics/parts/R48-IR12.html

(can be bought at www.conrad.nl)

ultrasound: ( from 10 cm to 255 cm )

SRF02 series

I had a problem with too stiff wires, and I had really to get the MODE pin HIGH to get it functioning
This script is showing how:
http://www.tigoe.net/pcomp/code/category/arduinowiring/39

Because it is I2C you can add this to other I2C devices on pins 4 and 5 analog of the Arduino

http://www.antratek.nl/Ultrasonic-sensors.html

There is an inexpensive one I have built from a kit:
(dutch: Ultrasoon Afstandsalarm)

Ultrasonic distance warning

www.conrad.nl article number 114456
but this is for warning between 10 - 80 cm, not really measuring distance.