Use windows paint!
zoom all the way in and set your pen to 1 pixel.. treat each pixel as a square in minecraft.. you can use it for side view or top view. best part? SHAPE tools in paint.
you can easily draw circles of controlled size and other shapes as well. best part, they render as anti-aliased lines into the 1 pixel boundary squares, giving you a really nice result to guide you in minecraft. just note that your character is only 2 squares high and the zoom in only goes so far so you may tend to draw HUGE structures at first. Very easy to make something 40 pixels tall but that is pretty damn big in game. This one video I saw also then used GIMP. Both software are free even.
http://www.youtube.com/watch?v=fETsa9kewdI