Turns out they're a form of sacred geometry, intended to lead the entire planet into a perfect, harmonious way of life (emphasis added):

According to science, crop circles don't exist.

Yet, as these examples demonstrate, crop circles, through their complicated geometric structure, are worthy of being ranked alongside the greatest creations of mankind. Perhaps even higher!

When analyzing crop circle forms through the precise and unalterable practice of sacred geometry one cannot help but appreciate that a mind of scholarly intelligence is involved, just as the great masters of Islamic and Egyptian art...

That these symbols are occurring primarily in wheat, the very symbol of the Earth Mother, is significant in itself. Perhaps they are here to draw us as a race together by this interaction with our symbol of life? Maybe their geometric symbolism acts as a mandala which like its Buddhist counterpart contains the very vibrations that aid one in achieving inner transformation at this most vulnerable moment in our history? ....

It is interesting that today we find ourselves at the final crossroads in our evolution. Since that day when we decided to abandon our faith in the universal way and follow the mechanical codes of science our consciousness has shifted from one of reverence for all things sacred to the worship of abstract materialism. Consequently, our change of attitude has endangered our living, breathing celestial sphere and it's perhaps why crop circles, with their foundations based squarely upon sacred geometry, have chosen to appear at this particular point in time, reminding us that if only we observe the fundamental laws of the universe we may still be in time to discover the secrets of universal harmony and salvage our very own symbol of eternal life, the Earth.

See? It just looks like "Hello Kitty!" to the uninformed.