Voip-Info.org - A wealth of information on VOIP

Kannel - An Open Source WAP and SMS Gateway

Kamailio - FAST, Rock Solid SIP router - (OpenSER)  This changes everything.

UMTSworld - mobile network information

Asterisk - Extremely powerful, production ready Open Source Telephony

OpenBTS - A fascinating open source BTS project

OpenGGSN - An Open Source GGSN

doubango - open source 3GPP IMS/LTE framework

OpenBSC - An Open Source GSM Network in a box

OpenSMPP - An Open Source SMPP Library

Harald Welte's blog