Basically the Saas vendors use applications from open source projects and then select certain models and offer them for use to the business community as supported programs
meaning that the applications are for anyone to use but you have to pay for the IT support.
(interested in free.... Philosophy of the GNU Project - GNU Project - Free Software Foundation (FSF)
Paragent uses a model of backoffice programs and certifies
it with IT support.
Nsite( most notorious) is another one in which Macy's department Stores uses.
SourceFire is into the security threat internet issues and certifies
Clam AV another open source project