Is there a website where I can plan a room layout?

[cupcakes]

What I'm thinking of is something where you type in dimensions and it gives you an image of how the completed room could look like. We are planning an extension and I am fed up of making plans to scale with little tables and sofas on scraps of grid paper. Is there an easier way? I know when we ordered our kitchen the shop had a programme where they constructed an image of what it would look like and I'm hoping to find something similar (for free...)

ikea has a kitchen planner that you download but you can set the dimensions of walls, position doors, windows and see it in 3d

Do you think it would enable me to do several rooms? The extension is going to be connected to two other rooms and the layouts of 4 rooms are going to be affected.