OpenBSC is a GSM network-in-a-box. A relatively short and non-technical introduction can be found at

News and an idea of what's happening in the developer community can be found on

To get started as a user, read Building_OpenBSC and check for howtos and getting started guides.

To get started as a developer, search for 'developer'. You'll find links to the mailing list, continuous integration server, source repositories, coding standards, etc.