GPRS is a technology used by mobile phones to transmit data. If you have internet access on your cellphone, it's likely that you are using GPRS. A project called OsmocomBB is underway to develop software that could be used in free mobile devices. Progress has been good, with regular telephone calls and text messages already working, but data connections are still not supported.

Cellular telephony seems to be the biggest network implementation (bigger than the Internet in means of user base) that has been surprisingly closed to inspection and experimentation despite its age. Lately, I've started actively getting informed about GSM network protocols and their free implementations such as OpenBTS and OpenBSC.