UI and drush command for core upgrades from D6 or D7 to D8 - the plan is for this to live in contrib for now and ultimately bring the UI into core (a decision will be made shortly before RC whether it will make it into 8.0.0) and the drush command into Drush itself.
After installing, visit /upgrade in your browser to access the interface.
Since the purpose of this module is to replicate the upgrade.php process it is designed to be as simple as possible. For this reason, it will attempt to run all relevant migrations known to the system.
Module developers may create migrations for contrib modules by creating custom migrate plugins. Talk to someone in #drupal-migrate for details or visit http://groups.drupal.org/imp.
As of August 12, 2015 the D6 migrations in core are essentially complete. Work on D7 migrations is currently underway - the goal is to have the big four (terms, users, nodes, comments) complete by the end of next week (i.e., August 22 or thereabouts).
To run a complete upgrade of your legacy Drupal site to the current D8 site:
drush migrate-upgrade --legacy-db-url=mysql://user:firstname.lastname@example.org/d6db --legacy-root=http://myd6site.com