Look at those things:
1. SER (SIP Express Router), www.iptel.org
A very popular SIP stateful proxy server with a lot of features (web interface, Radius AAA and many others) and a C-style scripting interface.
2. Asterisk - it is an IP PBX, fully supporting SIP. Indeed SIP is the "native" (the most widely used with) protocol for Asterisk.
3. The Vocal SIP stack. I think the website is www.vovida.org