Here is a video of a "prototype" tractor that was (is?) being developed by a defunct John Deere subsidiary. It looks a lot like some kind of insect with six legs or a lot like that mondo spider video we posted the other day. According to some of the comments with the video they were developing it for use in low impact logging but it really moves very slow. I can't see it being practical moving that slow (though that mondo spider moved pretty fast). There are three different videos of this from different angles and at different locations so check out the "related" section on the host site for the other two.