As a result of StumbleUpon's newest update, a lot of folks are confused about how it's integrating into the Google SERPs. There are two new features, one of which actually isn't a new feature, but it looks like StumbleUpon has finally brought these options to the attention of its users.
Here's an example of what you see with the feature turned on fully. Don't ask me why they categorized one of these as "Car Parts."
It appears that it's just a matter of a recent update -- nothing more, nothing less. In fact, you can turn it off here:
StumbleUpon Toolbar > Tools > Toolbar Options > Configuration Tab > Uncheck "Highlight Recommended Search Results," "Show Names of Friends Who Like Search Results," and "Show Topics of Search Results." At that point, the page will look like it used to -- but you'll still see a "See StumbleUpon Friends Reviews."
Of course, many people are a bit concerned about the privacy implications. I don't mind; I'm a big SU user.
You can disable just about everything, which essentially restores your search results to the normal results you'd expect.
In any event, forum discussion continues at WebmasterWorld.