I've used a SICK LMS-291 which is a laser ranging device. It runs around $6K and is superb. It can be configured to scan continuously and report the object distances for a 180 degree field of view at 0.5 degree increments - good out to about 40 meters, and as close as a few centimeters, with very good precision - millimeter accuracy.
It is a very, very, nice sensor, though somewhat heavy and power hungry. You need a beefy 'bot to carry it.
-Brian