Depending on your point of view, StumbleUpon is either a neat way to while away some time when you've nothing better to do, or a fantastic way of building your knowledge and awareness of great web-based information sources. Or, of course, a bit of both. Its role is to trawl the web on your behalf for stuff that meets your interests, and present these to you for your watching and reading delight.
Stuff is identified according to broad interests which you select from a list. As you tell the app whether you like or don't like what it offers you, it learns about your preferences and refines its way of working. That means StumbleUpon needs a bit of energy put into it during the early days.
The user interface is very nicely designed, and of course you can create lists of stuff you want to save, look at other people's lists, and share stuff you like too. Both the Android and iPhone apps let you share via Facebook, Twitter, email, or to another StumbleUpon user, and a new addition to the apps side of things is sharing via SMS.
OS: Android, iOS