So, here's a roundup of some of the most popular, most downloaded, or most innovative maps released in the last week, starting with the:

Singleplayer
Sphere of Roundness by Lobster
This 1940s map really explores the wide-open spaces of those old testing spheres. Considered simple and small by Lobster, but if this is him warming up, I can't wait to see what he has in store.

Cassata Chamber 1 by CassataGames
The first Portal 2 map out of Cassata, who you may remember from his days making Portal 1 maps. This very much modern-era test chamber runs you around a relatively small room, solving puzzles all over before finally earning the right to leave.

The Unknown by KLJF22
One of the first maps that really tries to put a strong narrative flavor in to single player, this is a heaping helping of map that'll have you puzzling and backtracking.

Multiplayer
Lots of Faith by ljdp
Our first Portal 2 Community Spotlight went to this map, a one-room co-op romp of faith plates, buttons, and robots going splat when the timing doesn't quite work out.

Halls of Menoetius by magicrat
One of the more divisive releases we've had so far, this "rubiks cube"-style puzzle doesn't require you to use the portal gun and instead needs only patience, and maybe a quickly-drawn diagram or two.

Co-op Alternative #1 by r1corix
A co-op map that has gotten somewhat mixed reviews from the community. Depending on who you ask, it is either a short-and-sweet co-op experience, or just too easy and boring. Give it a shot and let r1corix know where you stand!

Surprise by DrummerB
A co-op map that's been getting good reviews. From the forum chatter, it sounds like you and your partner have to escape from cages before you can really solve the puzzle.

And finally, our honorable mention:

Portal 1 Singleplayer
Take-Off by Sword of Apollo
This sweeping Portal 1 map is worth firing up the first game for. As though the core map itself, with its own difficulty curve and bonus-size length wasn't enough, the map includes challenges. This map almost certainly has more time invested in it than all the others here combined, and you should definitely take a look.

Tools
For all you Mac players out there, just this morning, zbakes figured out that CyCroniCa's Custom Maps to Extra utility will run on OSX, assuming you have Java installed.

This is hardly an exhaustive list of all the maps released this week, but if you're looking for somewhere to start in catching up, these are excellent choices. Did I miss something? Reply and let me know what maps you think are the best from the past week.

Edit:
Okokokok, here's my verdict:

Sphere of Roundness by Lobster
I had a lot of fun with this one. It's very well engineered both in terms of aesthetics and gameplay. The difficulty to me felt very similar to Portal 2 (which some say was too easy) but I enjoyed it.

Cassata Chamber 1 by CassataGames
This map was a little more difficult than Sphere of Roundness but not overly so. People looking for a challenge after playing Portal 2 should take a look. Aesthetically, it sticks to the "clean" Portal 2 theme, but there's a subtle undercurrent of Portal 1 in the level's design that gives it a kind of nostalgic touch (for me, anyway).

The Unknown by KLJF22
This map was something different entirely. As Hober hinted at in the OP, it makes use of backtracking which I'be always felt is good design sense as long as it isn't over-done. I personally found this to be the most difficult map of the three, but for the wrong reasons. It didn't take me a whole lot of thought to discern the solution to each puzzle, but sometimes executing said solution was unnecessarily difficult which made the map frustrating at times. This was particularly evident to me on the last puzzle, which was a bitch to set up correctly. One thing that took me a minute or two to figure out is that there are portal cleansers with no visible textures which agitated me. The visual design needs a bit of work and felt kind of plain and uninteresting in some places but all things considered, this is a good map (especially for a first creation). I like that it tries to take things in a different direction than most Portal maps and I think that with a little more polish, KLJF22 has the potential to create some really great stuff in the future. This map is definately worth playing, just don't be surprised if you get frustrated with it here and there.

One thing you didn't mention, though, were the secrets. My map has lots of custom-made stuff to find.

Anyway, good job everybody, getting those Portal 2 maps out so quickly.

And to repeat Hober in the first post, let us know if you think a map deserves to be given attention. We want to make sure the best maps can shine!

Co-op Alternative #1 by r1corix

Thanks you ;]
Yes, maybe it too strange, but I can consider all wishes and rectify all errors.
Also, ALTERNATIVE #2 50% done ;]

a_underground_test
Good looking, several (easy) puzzles and one vent (!).

Combustible Lemons
Ugly map with one interesting funnel puzzle.

Identity 00-02
Great visuals a la P2 beginning. Puzzles are just frustrating jumps though.