What tools do you use to help you build structures? [closed]

When looking at Minecraft pictures, I see that lots of people build lots of really neat structures, but I really wonder how they build some structures. Are the using some external tool to help them plan?

For example, let's take the case of a simple sphere, dome, or arch. How do you figure out where the blocks should be placed? Do you simulate the structure in some other tool? Do you use a calculator and the appropriate equations?

Solution 1:

If you want to make circles, spheres, or toruses (donuts) by hand, use some pixel templates like this one:

To build a sphere for example (one method of many), pick the diameter you want and build up circles as layers. For example, if you want to build a sphere with diameter 21, you would

1. Start with a circle of diameter 7 on the bottom (according to the preceding template, some can vary), as the first "slice" up from the bottom is 7 across.
2. The next slice is 11 across, so you would build a circle of diameter 11 on top, with the same center.
3. Continue on, building circles of diameters 13, 15, 17, 19, 19, 21, 21, 21, and another 21 would be the equator of the sphere.
4. That would make a hemisphere, continue on to make the full sphere.

Solution 2:

There's a map generator that specifically does that called Planetoids. It was featured on the Yogscast: