At one point people were talking about StumbleUpon clones without finding any, and the closest thing I've seen is http://www.siteshuffle.com which has a few similarities. It's like a homepage that lists your favorite sites, and webmasters can add a little "add to siteshuffle" button on their blogs. But how many users can we expect to be using SiteShuffle? And how else can we market through it??