Session Initiation Protocol (SIP)

FAQs, background materials and lists of current work in this field.
http://www.cs.columbia.edu/sip/

The SIP Center

Portal with technical and market resources aimed at commercial developers. Also provides a testbed area for software and proxy development.
http://www.sipcenter.com/

pulver.com

A collection of information on the protocol. Contains news, reports and an IP telephony job noticeboard.
http://www.pulver.com/

Sipcall

A UK based service provider. Also contains hardware related links and protocol white-papers.
http://www.sipcall.co.uk/

Soft Front

Provides a development environment for software centered around SIP/VoIP technology.
http://www.softfront.co.jp/en/

SIP Forum

A membership based forum. Also contains a news board.
http://www.sipforum.org/

PROTOS Test Suite

A test-suite to find vulnerabilities in protocol software.
http://www.ee.oulu.fi/research/ouspg/protos/testing/c07/sip/

SIPquest Inc.

Software component developer for service providers and telecom equipment vendors.
http://www.sipquest.com/

Brekeke Software

Products for small to medium sized offices and a server database.
http://www.brekeke.com/

Parrish Systems

Sells off the shelf code and provides a software consultancy service.
http://www.parrishsys.com

Interactive Intelligence, Inc.

Developers of a proxy server. Site contains downloads, documentation and a forum.
http://www.inin.com/sipproxy

Ninetel USA

A USA based proxy service provider and supplier of hardware phones and routers. Site contains call rates and service details.
http://www.ninetelusa.com

Free IP Call

A proxy service provider with information in English and French.
http://www.freeipcall.com

SIP411 Directory Services

A Yellow Pages style directory of SIP addresses.
http://www.sip411.com

VoIP News

Information on available technologies and software/hardware product reviews.
http://www.voip-news.com

PC Phone Review

News and reviews of hardware, software and service providers in the United States.
http://www.pcphonereview.com

BaseVoice

Commercial software development company that produces PBX, router, proxy and application server software.
http://www.basevoice.com

VoIP User

Consumer guide and forum. Also has software phone downloads and links to service providers.
http://www.voipuser.org

SIP Wiki

Wiki based resource containing information and links.
http://www.toyz.org/cgi-bin/sipwiki.cgi

SIP Compose Group

A working group experimenting and documenting the protocol. Contains various internet technical resources.
http://compose.labri.fr/documentation/sip/

Voice System

Design, consultancy and development for SIP Express Router (SER) servers.
http://www.voice-sistem.ro

LignUp Corp

Applications developer. Products include software PBX's for enterprise and and service providers.
http://www.lignup.com

Tech Invite

Portal containing links to technical information and RFC documentation.
http://www.tech-invite.com

SIPfoundry

An international open source community providing VoIP solutions and technology based on the SIP standard.
http://www.sipfoundry.org/

OpenSER

A branch of the SIP Express Router project, offering an open-source SIP server.
http://openser.org/