SIP-Related Web Sites

VoIP RFC Watch: number of lines and pages in VoIP-related RFCs
SIP Center: test tools, articles, general information
SIP Forum
SIP Happens is no longer a SIP site.
SIPdev: "your complete source for SIP development information. Complete with RFCs, APIs SIP-centered community forums, we aim to be your one stop for SIP development."
dmoz open directory
VoIP User, with a UK perspective
VoIP Resources, with (somewhat random) links to service providers, basic information and phones
netphonedirectory: directory of PC-to-PC and PC-to-phone services, including configuration information for various SIP ATAs