Fingers. Keyboard. Google. Professional Kitchen and Bath Designers: ) Failing that, a tape measure and 1/4 inch graph paper works really well.

Magicplan. Great app. You can measure the room with a camera or measure by hand and use the square room template to drag the dimensions to match your room. Great tutorials also. You choose fixtures from a menu, set the fixture or vanity dimensions and drag and drop them. Then you can view the whole thing as a 3D drawing. I took my iPad to the showroom so they had a good idea of what I was doing and how much space I had.

Graph paper and a #2 pencil work the best.

Magic Plan is a great app if you need something quick. Of course it is always better to have a professional come out and do it!