Neighbor Discovery (ND) is an enabler for communication in Wireless Sensor Networks (WSN). Attacks against ND, notably the well-known and simple to mount wormhole attack, can severely degrade performance and security. Hence it should be thwarted in any WSN deployed in a potentially hostile environment. To this end, we design and analyze a secure neighbor discovery scheme tailored for low-cost and hardware-constrained WSN. Moreover, we provide and evaluate a first proof-of-concept implementation of such a scheme for an off-the-shelf WSN: the Cricket Motes. Our scheme utilizes the ultrasound ranging capabilities of these motes, together with simple geometric tests, to effectively thwart wormhole attacks. It is provably secure against a classical 2-end wormhole attack, and raises significantly the difficulty of mounting a k-end attack (with k>2): To have a non-negligible chance of success, the adversary needs to know the locations of all nodes affected by the attack.