http://www.ipkall.com/
Offers free incoming Washington state SIP based numbers. Apparently their revenue model is the ~1c termination fees they make. No customer service is provided. Interesting.
http://www.asterisk.org/
An open source PBX based on Linux used as a building block in most voice on linux systems.
http://www.star2billing.com/
The commercial arm of a2billing, offering installation, support, and hosting.
http://www.asterisk2billing.org/
a2billing is built on top of Asterisk and handles all kinds of billing requirements. Prepaid calling cards, post-paid, CLI authenticated, etc, etc. Very powerful. Commercial support available.
http://gizmo5.com/pc/opensky/
OpenSky is a voip to skype link, provided for free by Gizmo. Supports calls and chat (I think) but probably not video. Very interesting...
http://www.mobivox.com/
Mobivox allows you to dial a local number and be connected to a long distance or international number for free or cheaply.