The OpenBTS project - an Free Open Source GSM

This is a project which plans to develop a open source application in Unix that allows any individuals to set up their own private GSM networks .
 
The Open BTS Project is an effort to construct an open-source Unix application that uses the USRP to present a GSM air interface ("Um") to a standard GSM handset and uses the Asterisk software PBX to connect calls. The combination of the ubiquitous GSM air interface with VoIP backhaul could form the basis of a new type of cellular network that could be deployed and operated at substantially lower cost than existing technologies in greenfields in the developing world.

Open Source Project at Sourceforge:
http://openbts.sourceforge.net/