VoIP blogger Linda Goin has taken the time and devoted the patience to gathering a linked list of 74 open source VoIP apps.
As she describes it, her list includes SIP, H.323, IAX, and RTP protocols, with clients, libraries, gatekeepers, and any other open source resource available for those specific protocols plus PBX and IVR platforms.
There's also faxware, voicemail apps, and middleware compliant to the stated protocols.
Better yet, these are not a mere collection of links. Each listed resource is accompanied with a knowledgeable yet quite understandable descriptive paragraph.
I know that "partysip" sounds like well, pouring from the keg and drinking down something cold, well, Linda shows us another way.
Partysip - Partysip is a modular application where some capabilities are added and removed through GPL plugins. Depending on the list of included plugins, partysip can be used as a SIP registrar, a SIP redirect server or statefull server, or a SIP service provider (game session, answering machine, etc.).
Great resource. Nice goin', Linda Goin.