Spencer started Digium as a tech-support company in 1999 for businesses using the Linux operating system. When his own company needed a phone solution, he created an open-source telephony program called Asterisk. By 2001, Asterisk had become the business, providing low-cost voice over IP access for both businesses and individuals.

Age: 29
Location: Huntsville, Ala.
Revenue: undisclosed
Employees: 50