The "EchoMeter Touch module turns your phone into a bat detector! " detector is available at

Site that reveiws bat (echolocation) detectors
This site has the most comprehensive review of available bat detectors.

How they work...
Please visit and support Bats NW. They have provided material and moral support for our Woodard Bay - Capitol Lake study.
Bat Conservation Int'l is the largest US organization working for bat conservation and promoting bats through education. At thier online store is a good collection of bat related articles (T-shirts, stickers, detectors, books...).