Moon halo
Tonight isn't the clearest night of the year despite being very chilly. It isn't good for observing as there is quite a lot of high-level wispy cloud and plenty of airplane contrails. However, I had just got on my bicycle to go home from work when I saw a pretty amazing sight in the sky. Luckily I wasn't going fast, so I managed to stop before I went into a nearby hedge. That'll teach me to look up at the night sky whilst cycling. But what did I see? There was the Moon in the east with Mars about 20 degrees or so away around to the right. Near Mars was something that looked a bit like a curved vapour trail. Looking more carefully I realised that it made a full loop of the Moon. This was no vapour trail but a Moon halo.

Labelled image showing a halo around the Moon - 16 November 2005. Click for a larger version. CREDIT: Stuart
The other feature of the photograph is a airplane contrail that was created as I was setting up my camera. If you look carefully, you can see that it even cast a shadow on the clouds below!
For great atmospheric effects such as 'upside down rainbows', supralateral arcs and pillars check out Les Cowley's site.