This robot by Oddbot is a great example of what can be done with IR light.
`The eye consist of 4 IR LEDs and 4 pairs of photo transistors. The phototransistor pairs are connected in parallel to increase their sensitivity. The phototransistor pairs are then connected to your analog inputs the same way you would connect an LDR. This circuit is really 4 FritsLDRs but using phototransistors instead of LDRs. The main reason for this is that the lens on the phototransistors makes them more sensitive to light directly in front of them and because LDRs are very slow to respond to changes in light.`