Nvil or Wings3d.  I downloaded the NVil trial when I heard people talking about it, and it's a very good replacement to Silo.  Actually superior to Silo in many ways, and seems stable.  Wings3d is no slouch either.  I like the sweep function, and some of the other tools in there are very advanced for a box modeller.  Hexagon is ok for the price, but having used an application with edge sliding, the workaround for that in Hexagon is barbaric.