I am so glad for Multiversity.
Multiverses are one of the greatest elements of literature, but so often they're poorly used. DC, in particular, seems to have a love/hate relationship with them. Depending on the era, they either love to have them but hate to use them, or love to use them but not want them around. So it's nice to see Grant Morrison using the crap out of them in Final Crisis, Action Comics, and especially Multiversity. And now with the Multiversity Guidebook that was released this month (January 2015), it's nice to finally see a complete map of all the worlds - minus seven - that is fleshed out enough that each could have its own line of comics if DC wanted. But it's two worlds in particular we're here for today, and the first one actually DOES have its own line of comics!
Earth-12 is home to the Batman Beyond 2.0 and Justice League Beyond 2.0 specifically, and more generally the DC Animated Universe. I've written about this world pretty extensively. This universe first appeared in Countdown #21 (December 2007) and then more fully in Batman Beyond 2.0 #1 (August 2013). It also made an appearance in Countdown: Arena, but the less said about that, the better.
The next Earth, however, is one I didn't expect to see:
This Earth is based on an alternate reality from the DC Animated Universe in which the Justice League has become a totalitarian police force. It was first seen in the Justice League episode "A Better World" (2003) and later mentioned in the comics, which you can read about here. Because these Justice Lords are based on the present-day Justice League and the Justice Lords we see at that link are based on the Beyond-era Justice League Unlimited, I'm pretty sure this is the first appearance of these characters outside of Lord Superman (although they could've appeared in flashback, I'm not sure).
And this is a good example of not being sure how close the New 52 multiverse would be to the 52 multiverse, because Earth-50 was originally home to WildStorm characters (which are now on Earth-0).