This was a huge 22 degree ring around the moon here in Dallas on Sunday night. I have seen small halos around the moon, but never anything as large as this! This type of ring is created by ice crystals that form in the upper atmosphere. All of the ice has the same crystaline structure and refracts the moonlight like a giant lens, creating a very impressive ring.
More info can be found here http://home.hiwaay.net/~krcool/Astro/moon/moonring/
I took these images in the park across the street from my house, using my Nikon D-80 and Nikor 10.5mm Fisheye lens. It is a 3 second F/7 exposure at ISO 800.