Sweet home 3D is a good program (CaoPaux has the link above ^^). It's pretty easy to use, and it comes with some furniture and other household shapes to stock the house with.
I used it to prove to Hubby that *his* idea for the room layout wouldn't work... in less than 15 minutes, I won... I mean, I had a room all drawn up with approximate furnishings.
The coolest part about the program is that you can change the viewpoint (overhead, slanted, etc) and rotate around the room any which way. It's a good program. And... the price is right.