They're a hoax. Apparently with some help, you can make a small one in about fifteen minutes:
http://en.wikipedia.org/wiki/Crop_circle
Still fascinating though. The ones above are works of art as far as I'm concerned. I mean even if these were man-made it doesn't make them any less impressive. How did people make such intricate patterns like these without aerial observation? It's not as if everyone who went out to make one had a balloon or something llike that on hand.
As for aliens, I guess it makes sense for them to exist. Although if we ever come into contact, I'll be very disappointed because I once wrote a short story/myth stating humans were the last of a once great universe of sentient species.