Today Microsoft released Windows 10 SDK Insider Preview build 17682, replacing build 17677, which was released a week ago. The new SDK has the same build number as the current Windows 10 client build that's available for Insiders in the Fast ring.
The main new feature in this SDK is support for MSIX, which puts apps in containers. To package an app with the MSIX format, you an use the MakeAppx tool, and all you have to do to install it is click on the MSIX file. They can run on any device running build 17682 or higher, but it's not currently supported by the App Certification Kit or the Microsoft Store (that should be added later).
Aside from the lack of support in the Store and the Certification Kit, the new SDK does have one other known issue to be aware of:
Missing Contract File
The contract Windows.System.SystemManagementContract is not included in this release. In order to access the following APIs, please use a previous Windows IoT extension SDK with your project.
This bug will be fixed in a future preview build of the SDK.
If you want to download the new SDK Preview, you can find it here.