This article from TechNewsWorld helps address the question: Can you run a business on free or open source software? The answer is highly dependent up the small business you run. I would say no, if you are heavily customer centric. I would say yes, if the work you perform is highly collaborative. I would say no, if your customers require up to the minute tracking of their purchases or service status.
One other type of free software I don't see mentioned here is the free but expandable (i.e. custom) for an hourly rate. If you were to grow attached to a particular piece of software and wanted just one add on (which soon become many), the developer will charge a small fee to customize the software. If the enhancement isn't good enough or common enough of request, your new enhanced version will not be part of the typical upgrade path, i.e. new releases mean additional fees to bring the software up to the latest revision. You can see the slippery slope here.
I also don't see operating systems mentioned here. Will the version you've grown fond of, migrate to the next release of the Microsoft, Linux or Mac operating system?
Besides the points I make above, the most important line to take from the article is as follows...
"it is definitely possible to run an entire small business on nothing but free applications. However, it requires good IT managers who are involved in the free and open source software communities and who look to project sponsors for good support..."