Today is the first day of Microsoft's Build 2018 developer conference, and during the day one keynote, the firm announced quite a few new features that are coming for developers. Coinciding with this, the company released Windows 10 SDK Preview build 17661.
It also happens to be the first SDK Insider Preview from the Redstone 5 development branch, which is the codename for the next feature update to Windows 10, due out this fall.
Microsoft hasn't yet published a blog post that describes what's new in the preview or coming in Redstone 5, but it's safe to assume that you'll be able to find some of the things that were announced at Build today. For example, Microsoft announced support for Fluent Design to Win32 apps, Visual Studio IntelliCode, Live Share, a Speech Devices SDK, Project Kinect for Azure, and much more. There's also additional support for building out Sets support in Win32 apps, among other things.
If you want to play with the new SDK Preview and explore a bit, you can download it here. Otherwise, Microsoft will be announcing it tomorrow.