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..

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..

The SIP Center - Portal with technical and market resources aimed at commercial developers. Also provides a testbed area for software and proxy development..

SIP411 Directory Services - A Yellow Pages style directory of SIP addresses..

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..

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..

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 User - Consumer guide and forum. Also has software phone downloads and links to service providers..