The Design Experiments plugin is a tool for prototyping designs within the context of WP-Admin. It contains a number of proof of concepts that the WordPress Design Team is exploring.
An important disclaimer: This plugin is called experimental for a reason. It is for testing concepts, and is not intended for use on a production site. These experiments may hide or visually break functionality in the admin area. Some of the experiments may break after a while, and will be removed from the plugin. Do not rely on this plugin for permanent changes to your site.