Through most of the history of Windows, the needed tools to build drivers have been free. I've always thought this was a terrific idea: If you want to support your device on Windows, then Microsoft will help (at least a bit) by making all the necessary tools available to you at no charge.
The driver tools haven't always been available for free, however. Back in the early days of Windows NT you had to have an MSDN subscription to get the (exactly right version of the) Visual C++ compiler. And, as most of you know, since the advent of the new Visual Studio integrated WDK you've needed to buy a copy of Visual Studio at the Professional or Ultimate level to build drivers.
But there's good news in Windows driver land today: Starting with WDK 8.1 Update which is available now the WDK works with Visual Studio Express 2013. That's the free version of Visual Studio. Yay! The tools to build Windows drivers are free once again for the community's use.
We should note that there are a small number of restrictions, however, on what's supported in the WDK with the free version of VS. When using the WDK with VS Express 2013, you can't convert projects from sources/dirs to native VS Solutions. Also, support for Code Analysis (otherwise known as Prefast and Prefast for Drivers) is very limited. You can read all about it here. But the important thing is that you can build drivers with it... real drivers... even drivers for ARM platforms.
Even with these small limitations, the community's ability to build drivers at no cost is great news for the community. Yay!