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.