OPAL is the "next generation" of OpenH323 that has a new architecture. Not only
does it support H.323 and SIP, but new VoIP protocols or devices can be added
very easily. It is being actively developed and is used by several projects
such as Ekiga (http://www.ekiga.org)

BTW: OPAL is an acronym for Open Phone Abstraction Library, but that is not
really important :)

WWW: http://www.voxgratia.org/