Roomeon 3D-Planner is a comprehensive interior design tool which allows you to create a 3D image of your room, then experiment with new windows, doors, flooring, furniture and more.
If you'd like to be accurate then you can start by drawing a floor plan for your room. But if a general idea is good enough, just choose a template which matches your room's general shape and you can get started right away.
Clicking the roomeon Objects tab then reveals an enormous library of items ready for inclusion in your room: fireplaces, lighting, radiators, carpets, rugs, beds, sofas, kitchens, and the list goes on. (These are generally real products, too: if you like something you can often follow a link to buy it.)
If you see something which you think fits in your room then simply drag and drop it onto the floorplan, the object will be downloaded, displayed, and you can try it out in different positions, see what works best. (The program does its best to help here, too, so if you're installing a new sofa, say, and push it towards a wall, it'll automatically orient itself correctly and snap into place.)
Roomeon offers multiple views to help give you the best possible idea of how your project will look. So you can place its camera inside the room, and "walk around" with the keyboard; the top down option gives you a bird's eye view of your project; or you might use a free floating camera which is able to check out your room from any angle.
Whatever you're doing, good control over lighting combined with the program's smart rendering means you can get photo-realistic images of how your room is going to look. An integrated screenshot feature makes it easy to share the results of your work, or you can even publish your design on the roomeon site, allowing others to examine it in detail.
You'll need to spend quite some time learning how this all works, of course. Roomeon may be easier to use than a full-scale CAD program, but there's still a huge amount of power here, and it takes time to figure everything out (especially as the interface is sometimes less than intuitive).
Still, it's extremely impressive for a free (for personal use) tool, and if you're planning a big interior design project then roomeon really could be very helpful.