The Windows Insider team has just announced the release of Windows Server 2019 Insider build 17666, which succeeds to the preview build 17650 released last month.
As a reminder, Windows Server 2019 will be the next release in Microsoft’s new Next Long-Term Servicing Channel (LTSC), and this version of the server OS should be generally available in the second half of 2018. Following the general availability of Windows Server version 1803 last week, Windows Insiders can also download today a new build of the next Windows Server Semi-Annual Channel release, which should be available later this year.
These latest builds of the server OS comes with a couple of new features, including performance history for Storage Spaces Direct as well as new cmdlets for simplifying the management of volumes with delimited allocation. ISO and VHDX files are available from the Windows Server Insider Preview download page, though please note that this build of Windows Server 2019 as well as all future preview builds will require will activation keys which you can find on the blog post.
In addition to these new Windows Server preview builds, Microsoft also released today the Windows 10 SDK preview build 17666, which you can download here. Microsoft has yet to publish a blog post to announce what's new in this release, so in the meantime you may want to keep an eye on the Windows Insider blog for more details.