A few nights ago, as I left my house and looked up to the sky, I was awestruck by a beautiful lunar halo. While I have observed this phenomenon before, this one was particularly stunning. These optical halos more frequently appear around the sun, and are usually caused by ice crystals in the atmosphere, reflecting and retracting light.
On one of my tramping trips up the Cobb Valley I spotted a 22° halo with two sun dogs. As I researched these phenomena further, I came across Paraselene.de with a comprehensive gallery of images with brief explanations dedicated to atmospheric optics and astrophotography—well worth a visit!
Another good reference is, of course, Wikipedia: Atmospheric optical phenomena.
Have you observed and taken photos of optical phenomena?