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.
An open source PBX based on Linux used as a building block in most voice on linux systems.
The commercial arm of a2billing, offering installation, support, and hosting.
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.
OpenSky is a voip to skype link, provided for free by Gizmo. Supports calls and chat (I think) but probably not video. Very interesting...
Mobivox allows you to dial a local number and be connected to a long distance or international number for free or cheaply.