Yes. Osmocom/OpenBSC actually started as an open-source "network in a box" which implements all critical GSM network elements in a single executable. It greatly expanded since then - check out a list of projects there. As I mentioned below, more companies are joining Osmocom and there will be hopefully more work on the core network side. Other open-source projects targeting core network elements are http://www.mobicents.org/ and http://www.projectclearwater.org/ (IMS).