You can download drupal-8.0-alpha10 or clone Drupal 8 in git:
git clone --branch 8.x http://git.drupal.org/project/drupal.git
You've probably heard that Drupal 8 is a major shift for Drupal, with lots more object-oriented code. If you're like me and "grew up" on Drupal paradigms, this might be a little scary! Thankfully, effulgentsia has taken the time to make Drupal 8's OO code less scary, and to show us what there is to love about it:
Drupal 8: Hello OOP, Hello world!