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