StumbleUpon recommends web pages based on pages you explicitly rate while the Google Toolbar recommendation feature learns from the implicit information in your Google search history to determine what kinds of web pages you seem to like. In my usage, both return a lot of misses and some hits, enough hits to be fun and occasionally useful exploration tools.
See also coverage by Inside Google ( ), Search Engine Land ( ), GigaOM ( ), SearchViews, and Google Blogoscoped.
Update: Mike Arrington notes that the Google feature is "strikingly similar" to StumbleUpon and John Battelle says:
The strategy ... [is to] have something in the wings, in case [they] don't win the acquisition game. This case is small - StumbleUpon. But from sources I've talked to, Google had built an entire Doubleclick killer, in case it did not win there.Update: Five months later, a BusinessWeek article claims eBay may use StumbleUpon "to recommend items to users based on their shopping history and the shopping histories of other people like them." I wonder if that feature would be in addition to or a replacement of the web page recommender toolbar.