It looks like Microsoft is delivering its promised more regular Windows builds. The company just pushed out a new build to Fast Track users of the Windows 10 Technical Preview: build number 10061.
Highlights of the new build are new Mail and Calendar apps, based on the same Universal Apps as the Windows 10 for phones apps released a couple of weeks ago, and more configurability of the operating system's appearance, including a new dark theme. The Continuum experience, used in tablet and hybrid devices, should also be improved.
As with the other Technical Preview builds of Windows 10, this isn't yet ready as a daily driver operating system, and Microsoft describes a number of known issues in its release notes. Perhaps most substantial is that traditional Win32 desktop applications cannot currently be launched from the Start menu. They can be launched from icons pinned on the taskbar, or from search, but not from the menu itself.