Update: Stacks 3 is out of beta and you can download the release here:
http://yourhead.com/stacks
You can also read all about all the features and on this nice blog post:
But we’ve left the Awesome Partials video right where it was. It’s still a great into to partials, so go have a watch:
Partials in Stacks 3
http://yourhead.com/partials/