Page Manager
Page Manager allows you to create custom pages, add blocks to them, and define different configurations (variants) for these pages based on specific conditions. It can also be used to override existing pages and change their content.
This is the Drupal 8 version of Page Manager, formerly part of the CTools module written by merlinofchaos
It supports the creation of new pages, and allows placing blocks within that page.
Like Drupal 7's Page Manager, it provides a concept of "page variants", each with their own selection conditions.
Additionally, it can be used to take over an existing page, like overriding /node/% to change what is displayed when viewing a node.
It utilizes the core Conditions, Context, and Blocks APIs to accomplish this.
In Drupal 7, Page Manager was the foundation of the Panels module.
The plans for the Drupal 8 Panels module have not been completely decided, but this module will not include any Layout GUI, and defers to the eventual D8 Panels module.
See this list for all core issues that were written for or required by this.