Sweet Home 3D hits the spot 3 ways - good software, easy to use, and free. It's open source, is updated regularly, and gets used by several architects I know and work with. About 100k downloads each week can't be too wrong. I've used it for finished product on Windows, Linux, and ML/Mavericks. It's on two of my Macs.
Just wanted to give a strong +1 for Sweet Home 3D. We are building a new home. Of course the architect is providing us with official drawings and renders. We use this to change furniture, textures, lights and such to get a feel for things before we commit. Really amazing quality for free software. An hour after downloading it I had the ground floor walls, windows and doors setup and was fooling around with positioning the 75” TV I hope to get. Kudos to everyone who helped make this app.