Was just looking through Final Crisis #7 earlier and it struck me how well a lot of the alt-Supermen there match up with the ones we saw in the Multiversity Guidebook. Pretty impressive for two comics published six years apart...
This is the first group shot we get of the Supermen of the Multiverse. Obviously that's Earth 5's Captain Marvel down front with The Question; behind them are (from left to right) Earth 35's Supremo, Earth 34's Savior and Earth 8's Hyperius. Behind them, again from left to right, are (I think) a pre-New 52 Ultraa (he appears later on in the issue with very red hair and I'm taking that as a decent sign that that's who he's supposed to be), next to him in the headgear might be Earth 11's Wondrous Man (maybe, not very sure at all about that as he kind of looks like the Guidebook version, but nothing at all like the version that apperared in Countdown - also he's supposed to be a Wonder Woman analogue, not a Superman one) and Earth 36's Optiman; and right at the back is Cyclotron from the T.H.U.N.D.E.R. Agents-esque Earth 39.
Things are maybe not so hot with the big double page spread (there are *a lot* of generic Supermen in the background here). All of the guys from the earlier group shot are here, and you might also be able to pick out the Superman of Earth 23, Sunshine Superman from Earth 47, Earth 10's Overman and Red Son Superman from Earth 30. A couple that weren't in the Guidebook for one reason or another are a goatee'd Superman way over on the far right, who I think is supposed to be Zod-as-Superman from Earth 15; and old man Superman, who I think is probably a decent bet for Earth Frank Miller's Man of Steel before they swapped that out for a bunch of post-apocalyptic pirates...
I think I see Steve Gerber's Omega the Unknown in between Savior and Captain Marvel there. Oh, and Apollo and Mister Majestic. Remember those guys??
If you can put a name to the face of any of the others let me know via the usual channels.
UPDATE: OK, so we can maybe put a few more names to loosely-inked background faces thanks to the Twitter hivemind. The blonde guy top centre looks like a pretty good bet for The High from Warren Ellis's Stormwatch run (thanks @PaulJonMilne!), and the Superman on the far right with the copious Silver Age neck flesh on display might be the New Frontier Superman from Earth 21 (thanks @DispenseroTruth!).
Also, in Superman Beyond #2, Zillo Valla mentions a bunch of alt-Supermen that missed out on the Ultima Thule's jaunt to Limbo - Savior, Guardsman, Icon and Principle. Earth 34's Savior makes it into this issue but none of the rest do. Guardsman is probably intended to be an alternate version of Marvel's Sentry, created by Paul Jenkins, Jae Lee and Rick Veitch, so it seems sensible to put him on Earth 8 alongside Hyperius.
Icon, created by Dwayne McDuffie and M. D. Bright, was the Superman of the Milestone comics imprint. When Final Crisis was published in 2008-9, DC were in the midst of attempting to fold the Milestone characters into their main Earth 0-centric publishing line but I believe that, like the T.H.U.N.D.E.R. Agents, that was part of a licensing arrangement with DC that has since expired.
Principle is likely a nod towards Bob Jacob, Gerard Jones, Len Strazewski, and Norm Breyfogle's Prime from the Malibu Ultraverse, and might well be the character I identified above as Ultraa (Prime also has reddish hair and a similar chest emblem). The Ultraverse characters are currently owned by Marvel, but they seem happy to let them rot in obscurity due to the possibility that they might (shock! horror!) actually have to pay the creators to use them under the terms they originally signed with Malibu. I'm going to go ahead and place Principle on the "anything goes" Earth 7, home to analogues of the Ultimate Marvel characters, a bunch of Dark Horse properties and Neil Gaiman's ex-Image now-Marvel character Angela.