StumbleUpon, as you may know, launched a big redesign for its desktop version (in addition to new apps for iOS and Windows 8). One of the big new features is Lists, which let the user collect different types of content in personalized lists which can be kept private or made public for others to enjoy (and Stumble).
Even more recently, they added Lists functionality to the toolbar, so users can add content to their lists (or create new lists) while browsing the web.
The company recently put out this quick tutorial video that shows you how to use it, so if you haven't messed around with the feature yet (now that it's out of beta), take a look:
As far as the toolbar goes, the Lists functionality is a welcome addition, even if another useful feature - the Explore Box - has gone away (it's still on StumbleUpon.com).