Previewed at WWDC, launching in the fall.
Apple Seeds First Beta of macOS Mojave to Developers
The new macOS Mojave beta can be downloaded from the Apple Developer Center, and once installed, subsequent betas will be available through the Software Update mechanism in the Mac App Store.
As this is brand new, early release software, it should not be installed on a primary machine.
macOS Mojave brings multiple new features to Apple's Mac lineup, including a new system-wide Dark Mode, an entirely revamped Mac App Store, and a Stacks feature for organizing files on the desktop.
Several iOS apps are coming to macOS Mojave, including Apple News, Stocks, Voice Memos, and Home, which will let you control your HomeKit devices on the Mac. As with iOS, Group FaceTime is available on the Mac with Mojave.
macOS Mojave is limited to registered developers at this time, but later in the summer, Apple plans to make a public macOS Mojave beta available, giving public beta testers a chance to try the software before it sees a public launch in the fall.