Windows DDK Version 3790.1830. It builds IA32, X64, and IPF code. DDK3790 is the last one that includes a set of tools for building 16-bit code. The 16-bit tools are still used by some EDK2 projects. To get them, you must run the DDK installer on a Win32 machine.
Windows Driver Kit (WDK) Version 7.1.0 (DDK 7600). This newer version builds IA32, X64, and IPF code. DDK7600 is not officially supported by EDK2 at this time, but it can be added easily. Just add it to BaseTools\Conf\tools_def.template by using the DDK 3790 section as a template.
Visual Studio Express editions. The recent express editions have a license that allows commercial use of the tools.