Session Initiation Protocol (SIP):FAQs, background materials and lists of current work in this field.
Brekeke Software: Products for small to medium sized offices and a server database.
FirstHand Technologies: Client and server software developer for service providers and telecom equipment vendors, to address enterprise mobility markets.
IETF SIP Charter: Working group for protocol development. Links to RFC's and internet drafts.
Parrish Systems: Sells off the shelf code and provides a software consultancy service.
PROTOS Test Suite: A test-suite to find vulnerabilities in protocol software.
pulver.com: A collection of information on the protocol. Contains news, reports and an IP telephony job noticeboard.
SIP Compose Group: A working group experimenting and documenting the protocol. Contains various internet technical resources.
SIP Forum: A membership based forum. Also contains a news board.
SIP Made Easy: An introductory tutorial talks about protocol functions, components, message types, how it works and where it is applied.
SIP Wiki: Wiki based resource containing information and links.
SIP411 Directory Services: A Yellow Pages style directory of SIP addresses.
SIPfoundry: An international open source community providing VoIP solutions and technology based on the SIP standard.
Soft Front: Provides a software development environment.
Tech Invite: Portal containing links to technical information and RFC documentation.
The SIP Center: Portal with technical and market resources aimed at commercial developers. Also provides a testbed area for software and proxy development.
Vidosystem: Produces softphones. Site has downloads section and information on protocols supported.
Voice System: Design, consultancy and development for SIP Express Router (SER) servers.
VoIP News: Information on available technologies and software/hardware product reviews.
VoIP User: Consumer guide and forum. Also has software phone downloads and links to service providers.