OpenBSC is a GSM network-in-a-box. A relatively short and non-technical introduction can be found at http://www.h-online.com/open/features/Building-a-GSM-network-with-open-source-1476745.html
News and an idea of what's happening in the developer community can be found on http://planet.osmocom.org/
To get started as a developer, search http://openbsc.osmocom.org/trac/wiki for 'developer'. You'll find links to the mailing list, continuous integration server, source repositories, coding standards, etc.