From Network World article.:

All these companies business model is to open source their software and charge for services.

Open Source companies to watch:

Cleversafe - Grid file system.

Digium - Open Source PBX - Asterix.

Hyperic - Platform for managing heterogeneous IT environments.

Optaros - Helping enterprises choose open source solutions.

Qlusters - System management platform.

rPath - Preconfigured, pretested application appliances that can be downloaded and deployed by enterprise users in minutes.

WSO2 - Tungsten, an Apache-based open source application server built from the ground up to handle Web services.

Sahana - Sri Lanka-based nonprofit committed to promoting open source software development.

Zenoss - Network and systems-monitoring software.

Zmanda - AMANDA backup utility software.