Photo: tiagomuller [Flickr]
Tom Davie of Environmental Graffiti wrote a very neat article about the 20 most intriguing light phenomena, covering such things as moonbow, fogbow, aurora, crepuscular rays and so on.
This one above is the 22° halo:
Halos are among the most well-known of optical phenomena and appear under a variety of guises. The most frequently scene is the 22º halo, caused by ice crystals in high altitude cirrus clouds, yet the particular shape and orientation of the crystals can create variation in the appearance of the halo. During very cold weather halos formed by crystals close to the ground reflect sunlight between them sending it in several directions at once, an effect known as diamond dust.
Link - Thanks Tom!