The old RTB, or this upcoming RTB 2.0?
Old RTB: Blockland. It was basically the beta Blockland. :D There were many different things like the EW (editor wand) which you could stretch bricks with. (to reduce lag) And movers with the wrench (made bricks move) and bricks could float, overlap, and such. It was pretty much the not so good, but also epic blockland.
RTB 2.0: Mod. look up some stuff for more.