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.
An open source PBX based on Linux used as a building block in most voice on linux systems.
Elastix combines Asterisk, HylaFAX, Openfire and Postfix and others into a "unified communication server" that handles telephone calls, email, instant messaging, fax, and more.
The commercial arm of a2billing, offering installation, support, and hosting.
TAS is Noom's company in Bangkok providing a range of IT services, along the same lines as NetSolutions Asia (Robert Horgan) who he used to work for. They focus on some open source stuff, asterisk, scalix, suse, etc.
Virtual phone systems aimed at web entrepreneurs. Includes voicemail to email, extensions, all that jazz.