Perhaps it's because I am working on a voxel-ish game, but Qubicle Constructor has proven itself to be my most fruitful asset scratchpad.

On the flip side, I don't tend to use the assets made in Qubicle as they are a bit vertex heavy - even with the Unity plugin. Usually I rebuild the meshes from scratch in Maya.

I just wish there was an iPad version. Hmm...