Earlier today, Apple seeded the second public beta build of iOS 11, and now, new builds are rolling out of macOS 10.13 High Sierra and tvOS 11. The build numbers are 17A306f and 15J5324f, respectively.
macOS High Sierra will include a number of improvements for the OS, one of which is the new Apple File System (APFS), which is completely 64-bit. Safari will now allow you to turn off autoplay videos on websites, which many users will surely appreciate. There are also a number of improvements to Mail and there are new editing tools in Photos.
tvOS 11, on the other hand, is much less significant of an update. While it does contain some new features, very few of them could be considered front-facing.
If you want to sign up for Apple's Public Beta Program, you can do so here. If you're already enrolled, you can update macOS through the Mac App Store, and you can update tvOS through the Settings app.