On the same day that the news breaks that eBay is acquiring StumbleUpon for $40ish million, Google announces that they are building strikingly similar functionality into their Toolbar. Google (along with AOL) were reportedly in the running to acquire StumbleUpon until very recently.
The Google Toolbar now includes a dice icon. Click it and you'll be taken to a new website that Google thinks you'll find interesting based on your previous search queries.
This isn't an exact duplicate of StumbleUpon functionality, which bases recommendations on your and other StumbleUpon users' votes of sites. But the end result is the same – serendipitous discovery of new and interesting websites based on your core interests.
Google also has a widget that can be added to their personalized home page with recommended sites.
Om Malik says this is Google lashing out at StumbleUpon and eBay because they lost the deal. Given the timing, I tend to agree.