StumbleUpon recently(?) added a "search result enhancement" feature to their toolbar. It adds StumbleUpon "star ratings" to Google results (See screenshot). This got me interested and I had a closer look at their extension...

As part of this "closer look" he came up with the code to return a general ranking for the URL you would give it. It does require a StumbleUpon auth key and username, but if you use the service already, you're one step ahead on that.