Hi Everyone!
This release includes a truly massive amount of changes, primarily due to a very successful sprint at DrupalCon LA, which helped to push a couple long-standing issues over the edge. Here are the highlights:
Drupal 7.37
It's in! Not much more to say. :-)
New default preview mode for the "Add Content" dialog
The previous default preview mode showed a preview for ALL the widgets in the selected category. With a small number of widgets this was fine (and looked pretty cool!) but had performance problems with a large number of widgets, not to mention some usability and accessibility issues.

So, we've added a new mode which only shows a single preview at a time.
The old mode still exists and the default won't be changed for existing sites (although, you can change it manually by visiting /admin/panopoly/settings/panopoly_magic and changing "Enable previews when adding panes" to "Single") but this will be the default for new sites going forward!
NOTE: The new mode is currently incompatible with Radix and Radix-based themes! Although, there are some patches in the works - check out #2496027: Generate preview markup in a theme function/template and #2502939: Incompatible with Panopoly's "Single" preview mode on the "Add content" modal. For the time-being, however, it's probably best to keep using the old preview mode if you use Radix.
Increased test coverage
One of the main focuses at the sprint at DrupalCon LA was increasing our test coverage, and thanks to all of the participants there, we were able to do just that in a number of important areas! The bigger our test coverage, the less likely it is that we'll break things when we make changes and release new versions, so this is super important work.
(BTW, if you want to contribute to Panopoly, one the best ways to do that it to help write tests! The effect on the project is long-term and the pay-off increases over time.)
Module updates
We've updated Fieldable Panels Panes, Menu Block, Media, Manualcrop and Apps to newer versions!
In particular, Fieldable Panels Panes is responsible for some of the core functionality of Panopoly, and this release (the first in almost 2 years!) represents a massive amount of work by the Panels community and the FPP maintainer, Damien McKenna.
Behat 3
Our Behat test suite previously relied on Behat 2 and we've finally managed to make the leap to Behat 3!
Since this affects all of our tests and our tests are constantly changing, this was a conversion that needed to start and finish in a very short period of time, and we were only able to pull this off largely thanks to a massive push that cboyden did over a 3-day weekend!
Accessibility improvements
A lot of work has been put into improving the accessibility of CTools and Panels (by Panopoly contributors, namely, cboyden) and those patches have been finally merged upstream and included in Panopoly. This plus the new preview mode make Panopoly more accessible than ever!
For more information, please see the full release notes:
https://www.drupal.org/node/2480509
Remember: you have to update the WHOLE profile - not just the individual panopoly_* modules! Otherwise, you'll be missing important module updates/patches, and it isn't guarenteed to work.
For full instructions on how to upgrade, please see the new Upgrading Panopoly guide! (We invite you to help us improve it as well.)
And, of course, you can find links to download, documentation and report problems on the Panopoly project page:
https://drupal.org/project/panopoly
Thanks!

Comments
Radix is currently incompatible with the new preview mode
Sorry this didn't get into the release notes originally, but I just added it there and in the post above.
The new preview mode is currently incompatible with Radix and Radix-based themes! But there are some patches in the works:
For the time-being, however, it's probably best to keep using the old preview mode if you use Radix.