Free Online 3D Room and Garden Planner

Sweet Home 3D is a free interior design application that helps you place your furniture on a house 2D plan, with a 3D preview.

Small Blue Printer Design your house plan blueprints online with it, then take a 3D walkthrough your design, check out an isometric view and print out your plan.

For more complex plans, with furniture, more rooms etc. Try FloorPlanner