VOIP: The Revolution
VOIP is another of those revolutions (like the Internet and the Web) that promises to empower the little guy but which has the potential to be taken over by big corporations. Which is why Microsoft has now discovered it and started selling it.
VOIP stands for Voice Over IP (where IP is Internet Protocol as in TCP/IP, a meaning IP had long before people started acronymphing Intellectual Property into IP). Basically you convert voice signals into data packets and send it like data over the internet. As with most such things, there are free, open-source protocols for VOIP, such as SIP and IAX, and there are non-free, undocumented private protocols such as that used by Skype. One of the leading free implementations of VOIP is a program called Asterisk, free from Digium (Digium sells hardware to go with Asterisk, and sells supported versions, so they do have a sustainable business model). There are a few GUI administration tools for Asterisk, including:
In addition to Asterisk, there are other freeware "Software PBX" offerings; see especially:
There are many consumer-oriented services such as Vonage and some of the "home phone" commercial offerings; I believe many of these also use the public SIP protocol, but they don't much document what they do.
For a quick summary of VOIP, check out this Toronto Asterisk Users' Group fanfold (pdf). For more information on Asterisk and the revolution, see the book Asterisk: The Future of Telephony (book download in PDF).
One of the more widely used proprietary protocols is Nortel's Unistim (universal voip stimulus?) protocol, used in e.g., their BCM systems. Although Unistim is not publicly well documented, it has been reverse-engineered to the extent that there is an Asterisk Channel Driver that lets you use the Nortel Unistim phones (i200x and i110) on Asterisk. I've partly written a user's guide for these Unistim phones (PDF; Creative Commons license). I also put some of the usage information on the O'Reilly eTel Wiki before they announced they were cancelling the eTel conference and winding down the main eTel web site. Note that the Unistim channel driver is (as of 2008-05) still not quite ready for prime time...