Sending/receiving data from multiple units

[system]

GPS tracker ?
http://www.pronto.com/user/search.do?SEM=true&query=arduino+gps+tracker&adid=b99960b9-a6ad-43a8-8da0-6b16b8c4d229-0-pt_msb&kwid=arduino%20gsm%20gps%20shield&cid=2858595103&loadingComplete=true&owl=true

GSM/GPS ?

SIM908 Arduino GPS/GPRS/GSM Shield - DFRobot

This GPS/GPRS/GSM shield uses an embedded SIM908 chip from SIMCom.Featuring an industry-standard interface and GPS function, the combination of both technologies allows goods, vehicles and people to be tracked seamlessly at any location and anytime with signal coverage.