After releasing beta 2 of iOS 10.3, watchOS 3.2, and tvOS 10.2 yesterday, Apple today has seeded the second Developer Beta of macOS 10.12.4 Sierra. Build 16E154a comes exactly two weeks after the first one.
The main new feature in the OS update is going to be Night Shift, which adjusts the blue light that's emitted from your screen at night, allowing you to get a better night's sleep. Siri will also be able to check cricket scores, schedules, and player rosters from the Indian Premier League and International Cricket Council. There's also dictation support for Shanghainese, and there are system-provided APIs for reading, writing, and navigating PDFs. The release notes did not specify anything that's new in this build.
Safari 10.1 beta 2 was also released. This is part of the macOS beta, but if you're still on OS X 10.10 or 10.11, it's offered as a separate download to test. Focus areas include general website compatibility, extension compatibility, debugging using Web Inspector, and HTTP Authentication.
As for those on the Public Beta program, you should see the new macOS beta today or tomorrow, as well as yesterday's iOS 10.3 build.