This compact sonar range finder by Maxbotix detects objects from 0 to 6.45 m with a resolution of 2.5 cm for distances beyond 15 cm. Unlike other sonar range finders, the LV-MaxSonar has virtually no dead zone: it can detect even small objects up to and touching the front sensor face!
More info:http://www.pololu.com/catalog/product/723