KAMAILIO (OpenSER) - robust, secure and scalable Open Source (GPL) SIP (RFC3261) server implementation with large features set (over 90 extension modules). As of May 2009, sourcecode is hosted by GIT repository at http://sip-router.org
...It supports click to dial and screen popping.
It is compatible with Freeswitch and babblevoice.com, but potentially should work with any SIP server which supports RFC 4235 (not Asterisk!).
Sourcecode now hosted at github. https://github.com/tinpotnick/bv-tapi.git