To Explore Further

Wikipedia VoIP
Concisely-written non-technical explanation of VoIP with good links.

VoIP Wiki
I used this and its linked pages a lot in writing this article. Lots of good information via its well-organized linked articles.

VoIP Howto
Roberto Arcomano's widely linked article is somewhat dated, but the Overview and Technical Info sections have clear, concise explanations of VoIP basics.

What is SIP?
Very understandable Network World article that will have you understanding SIP's pieces and how they play together in no time.

Getting Started with SIP
Another Network World piece that describes how to get a feel for how SIP works and how to work it into your existing PSTN setup if you're a small business or just play with it if you like to tinker.

VoIP Service Provider List
Community-maintained list of VoIP telephony providers and most likely to be up-to-date. Doesn't consistently provide info on the protocols supported by each provider.

SIP service providers
Not a huge list, but at least you know they all support SIP!