A New business model
Asterisk is developed using the Open Source software development method.  Originally created by Digium, a maker of line termination cards, Asterisk is not restricted to use with Digium hardware, though its popularity has certainly promoted the sale of Digium boards.  Because the source code for Asterisk is available, anyone can add support for their hardware or software, fix bugs that impact their business, or add features that they need.
"Open Source Marketplace"
For those who don't want to "get under the hood" the openness in the Open Source community means that their are large numbers of vendors competing to support you.  Many of the TAUG members in Toronto offer local commercial deployment, support and development services.  As you might imagine, Digium also offers commercial support packages to give you access to their core expertise. 