American game, EEs, Buttercups, Old english games (Oxfords), i think some oriental gamefowl, and basically any birds when you cross yellow legs with blue legs (or that's how i've heard green legs are "created")..
I have Pyncheon Bantams...They have WILLOW legs (green).....any other color to DQ at a show. Polish with any color leg other than blue slate are crossed or not pure. EE's can have pretty much any color of leg or egg...for that matter. They are NOT Standard bred birds and you never really know what's going to hatch from them. Buttercups and a few rarer European breeds do have Willow legs.
Some Seramas have green legs, although breeders are suppossed to be steering away from them, in favor of yellow or swarthy. In this breed, it seems to be passed on like sex-linked recessive.