working in big U.S. companies, I've come to realize that the big advantage of Open Source software isn't price, it isn't freedom (the principle of the thing, etc), rather, it's the perfect match between the terms/costs of adopting OSS and the needs of business. With proprietary software, the best you can hope for is that you can get a free download/trial version, do some work, and then make a somewhat informed decision whether it's worth pulling the trigger for expensive licenses, etc. After which there is the never-ending trauma of budgeting, creating business cases, adjustments to budget plans, dealing with purchasing departments and procedures...bleah. The hassles don't stop with approval to purchase, you have to manage all those licenses, deal with fake-tan account managers who are always trying to upsell, and countless other annoyances.

Open Source Software on the other hand fits the needs of a business like a hands fit a glove. Technical staff can download a fully functional piece of software *today* and start work. If they don't like OSS app server X after an afternoon's work, they can download OSS app server Y and try that. Once a business has found the sweet spot, a piece of Open source software that works well for them, they can just keep using it, copying tar files to different environments, and focus on solving technical problems to meet business needs, not solving socio-political problems to meet internal process needs (budget fun). Further, if things really get rolling and a piece of OSS software is widely deployed, a business can contract for exactly the support it needs. MySQL is a prime example of this, at work (Fortune 500 company) we acquired MySQL formal support, and they are just outstanding. The contrast between support from Microsoft and Oracle, and support from MySQL, is genuinely amazing. For relatively little cost, we can call a MySQL engineer, not a fake-tan account rep, who will go as deep as we want into an issue.

So the terms of use and costs of proprietary software caters to the needs of the software vendors, while terms and costs of OSS cater to the needs of the end user. That's basically it.