Asterisk (discovered via this Jon Udell post) is a complete PBX in software. It runs on Linux and provides all of the features you would expect from a PBX and more.
Quoting from the Asterisk home page:
Asterisk does voice over IP in three protocols, and can interoperate with almost all standards-based telephony equipment using relatively inexpensive hardware.
Asterisk provides Voicemail services with Directory, Call Conferencing, Interactive Voice Response, Call Queuing. It has support for three-way calling, caller ID services, ADSI, SIP and H.323 (as both client and gateway).
Check the Features section for a more complete list.
Naturally, this got me thinking about Skype; could Asterik and Skpe be meshed into something out of this world (Sykype, WiFi, and Asterisk)? But Skype isn't Open Source, so this would have to be something the Skype team would want to embark upon I guess?
As I pondered I searched further, and stumbled across this Open Source competitor of