We've covered a ton of floor planners on Webware, but none of them have been as cool as MyDeco. The tool throws in the usual assortment of tables, chairs, beds, and other living accouterments that you can price, purchase, and move around in a customizable space.
The tool lets you emulate single rooms or an entire house by plugging in the dimensions and adding walls, windows, and doors. It also manages to give you a 3D view of what those items will look like in that space, not just from the top down like most floor planners. In this mode you can zoom around, or pinpoint any angles using a virtual camera and take snapshots to send to your cohabitants. It's just plain cool.
I don't think it's really a perfect way to simulate the effects of moving actual furniture, but in about 10 minutes I had put together a pretty slick looking pad. Like I mentioned before you can actually purchase items you like through the tool's sister site, although it's based in the UK, meaning if you don't live there you'll have to track down the local counterparts.
Furniture juggernaut IKEA has its own tools that are a little similar but they're separated into kitchen, bedroom, and office, and must be downloaded and installed on your machine. In that regard, MyDeco's got the upper hand since it will run right in your browser as long as you've got Flash installed.