Update: Microsoft is forcing users to download the ISO file using the Media Creation Tool. If you still want to download the ISO file directly, you can do so by changing the User Agent String in your web browser. This will make the browser pretend its on an operating system such as Linux or macOS, which Microsoft permits direct downloads from.
If you need to download Windows 10 on an alternative operating system such as OS X (MacOS) or Linux; the Media Creation Tool or Windows 10 Upgrade Assistant will not work because those applications are executables. Previously, Microsoft provided the Techbench website for such as scenarios. The web page has now been redirected to the Software Download page. So if you are looking for it, that's where you will now find it.
Scroll down, click in the list box and choose your edition. Make sure you choose the right version. Click Confirm then wait while the page is verified.
Windows 10 - contains Windows 10 Pro and Windows 10 Home
Windows 10 N - only for customers in Europe, contains Windows 10 Pro N and Windows 10 Home N
Windows 10 Single Language - for users running Windows 10 Single Language, Windows 8 Core Single Language, Windows 8.1 Core Single Language or Windows 8.1 with Bing.
Choose your language then click Confirm. Wait while the page is verified.
Choose the architecture you wish to download. Remember, you only have 24 hours to download the ISO before it expires. When you are done, use your favorite DVD burning utility or mount it in a Virtual Machine.