SIP

FirstHand Technologies
Client and server software developer for service providers and telecom equipment vendors, to address enterprise mobility markets.
http://www.firsthandtech.com/

IETF SIP Charter
Working group for protocol development. Links to RFC's and internet drafts.
http://www.ietf.org/html.charters/sip-charter.html

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

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

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/

SIP411 Directory Services
A Yellow Pages style directory of SIP addresses.
http://www.sip411.com

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

SIP Made Easy
An introductory tutorial talks about protocol functions, components, message types, how it works and where it is applied.
http://geocities.com/intro_to_multimedia/SIP/

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

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

Soft Front
Provides a software development environment.
http://www.softfront.co.jp/en/

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

Vidosystem
Produces softphones. Site has downloads section and information on protocols supported.
http://www.vidosystem.com

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

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