SIP Introduction The SIP Introduction is a practical introduction to the SIP protocol, including visual displays of how the protocol works.
SIP Tutorial A comprehensive tutorial to the SIP protocol (downloadable in Adobe Acrobat format).
Product Database 
The product database lists SIP-relatedi  products. Any user or manufacturer can add a product to the database .
List of free SIP software provided by iptel.org and third parties.
Other Useful Sites
- SIP site  --
maintained by SIP author, Henning Schulzrinne
- http://www.tech-invite.com/  -- excellent website with many detailed
information including call-flows
- Packetizer's SIP
site  (very useful, still use with caution -- website maintained by advocates of
formerly competing technology, H.323)
- SIP Phoenix
Group  --A good site with different categories of research papers on
- Voip-Info  -- large wiki around all topics VoIPi