Sourcefire does almost exactly that. Their software is open source and you can download and install it for free.
They sell licenses and services, and you can pay for quicker updates to firewall rules and the like. As commercial customers, you can have their software engineers onsite tuning things to your environment, etc.
Covalent is a company that sells Apache. Obviously, they didn't manufacture Apache webservers, but, like Xamp or Wamp, they sell value added bundles that include Apache configurations pre-compiled with PHP or Perl or what have you.
Again, they also have a services division, but I don't personally know much about it.
For the record, no, my company does not sell GPL software. It was proffered as an example that non-GPL-based companies rely on both software sales and services, and it doesn't really matter whether or not the software is GPLd. I don't know why you'd hold it against RedHat that their software is freely downloadable when they're still selling more than a few copies of RHEL.