StumbleUpon, get some lawyers and go after them or just have a beer summit ;)
From the page: "Could they try to be more blatant in ripping off StumbleUpon? The toolbar has a random Stumble-like shuffle button which randomly takes you through different Tumblogs in a similar way that StumbleUpon窶s toolbars do."
Possibly a publicity stunt to get press?
are you serious? http://su.pr/1zf5D7
If you ask me, the best would be StumbleThru Tumblr via StumbleUpon toolbar, the same way as YouTube, MySpace, Flickr, etc. That would be cool!