One of my favourite pictures from NASA's
Astronomy Picture of the Day happens to be an excellent sky shot.
Explained at
http://antwrp.gsfc.nasa.gov/apod/ap020926.html with a larger version available if you click the image. The APOD
Archive has thousands of beautiful pictures of space, the sky and related stuff. Though this one is definitely my favourite sky shot. I think of it whenever people bring up clouds, because this rocket trail is just so gorgeous, it's hard to believe it's real. ^^