I have an example of using the Devantech SRF04 sonar range-finder with PBP in our
Micro-Bot projects section here http://www.rentron.com/Micro-Bot/Sonar.htm

Devantech makes the SRF04 schematic & source code available for folks that prefer to build
one themselves: http://www.robot-electronics.co.uk/htm/srf1.shtml

They have several more here: http://www.robot-electronics.co.uk/s...angers1999.htm

The Parallax PING module linked to by mackrackit should also be pretty simple to use.