This tutorial describes simplest possible IR obstacle avoiding sensor. Bumper can tell, if there is an obstacle in front of it, but it can not measure distance. It is very easy to construct and use, so it is perfect for begginers. Also it is very cheap, ideal for budget solutions. Simple AVR program and schematics included.
P.S. Sorry for a pdf file - I am not very fimiliar with html, especially image editing, but web version is coming soon.
P.P.S. Sorry for some content trouble not long ago. It was related with web edition.