Android 6.0 SDK is now available
And on the hardware front, Google added download links to Marshmallow Preview hardware system images. But, before you let your hopes balloon into stratospheric proportions, be aware that Google's only made a few ROMs available, and designed for just a handful of Nexus devices (5, 6, 9, Player).
That being said, if you are a programmer still anticipating on getting your hands on the SDK as soon as humanly possible, you're in luck: open Android Studio, select Configure, then SDK Manager, check the SDK 6.0 box, and install. That's it.
If you are comfortable using terminal, you can start SDK Manager with the android command (found under tools):
# android
And finally, to update the non-gui way, use this command:# tools/android update sdk --no-ui