Google have released SketchUp, a free drawing program for Windows. Like Writely, SketchUp was a commercial product before being acquired by Google. But SketchUp is a lot more mature a product than Writely.
With SketchUp you could design a house. But it may be a bit more than the average dabbler needs, or can use.
Google also provide a "3d warehouse" of pre-prepared designs in SketchUp. So if I wanted to design an apartment block from shipping containers, I can download a shipping container from the warehouse, to start.