Autodesk Dragonfly does easy, web-based home design
There's not much of a learning curve involved - click, drag, rotate, resize - it's all very intuitive. If you've played The Sims at all, you'll be right at home with Dragonfly. No, it's not a commercial-strength app like AutoCad, but it's at least as good as any of the boxed "3D home designer" programs I've played around with in the past.
Dragonfly makes arranging things easy by snapping to a grid and displaying guides to let you know when you're lined up with other objects in the room. To help align walls, doors, or windows, measurements are displayed when you add or modify an item.
There are loads of architectural and design elements to choose from, all categorized and fully searchable. Register for an account and you'll be able to save your designs and export them to JPG or DWG (you know, in case you want to ship it off to your architect or engineer).
While it's easy to use, Dragonfly is surprisingly powerful. It would be even better with some landscape design features, but who knows - perhaps that's something AutoDesk will add in the future.