Paint, if you don't want to shell out cash or steal, CAD if you're okay with either of those. Beyond that, I don't know.
I know some basics of drafting a floor plan, but are you speaking of something for an RP or you want to actually design a legitimate floor plan for construction purposes? Paint will get the point across, as long as you and whoever is looking knows the symbols. (For example, a door isn't a space in the wall, it's actually includes a line that follows the arc of the door as it opens/closes. Windows are like rectangles on the wall lines, since it's alway a bird eye's view. If you don't have a door, then yes, there is just a stop to the wall. Circles are used for pipe outlets, rectangles for grates. Drawing on electricity and water lines gets more complicated, but you can easily show it if you color code the wires and draw in the lines.)