There are two that seem to be the most common:
- Develop free software and sell support for it
- Develop free software and offer it under a less restrictive license for a fee.
The second model mandates that the company own 100% of the copyright of the software. This means, contributors are required to sign a copyright assignment.
Here is a hypothetical application for the second:
"Acme, Incorporated developed a feature rich, aesthetically pleasing software SIP phone and released it under version 3 of the GNU Affero General Public License.
Optionally, companies interested in re-packaging / re-branding the phone can pay Acme to use the standard GPL3, LGPL or even less restrictive licenses to establish a proprietary fork of the project.
Acme, Inc. was later purchased by Yoyodyne Systems, who paid a lot of money to acquire full ownership of the code"
I'm not going to go into the idealistic merits of either scenario, I'm just presenting them as the ones I'm most familiar with.