Does anyone know of a website where you can given the dimensions of your bathroom and see how they would arrange your tub, shower, vanities and toliet?
If you're a little computer saavy, give Google Sketchup a try. It's a free basic (but powerful) 3-D design program. What's nice about it is that it's connecting to the Google object library where you can download all sorts of already pre-built objects that people have already made.
@ratchet left it as a comment, but for almost all these types of assessment I use paper cutouts. It is very simple to sketch the physical dimensions of a room on graph paper and then move cutouts around. This lets you get a very good assessment of floor clearances etc.
Where it lacks is in 3D visualisations so if you want that, you will need to go for Sketchup, Sweet Home 3D or one of the other 3D apps - they are trickier to use but the learning curve isn't too bad if you already have computer experience.
It is the sort of thing that should lend itself well to tablets, but I haven't seen anything that is intuitive and user friendly on a computer yet.