A set of Photoshop & GIMP brushes composed of various types of clouds. Some of the wispy ones would work very well for creating mist if set at a lower opacity.
We also have a set of High Resolution Clouds Photoshop & GIMP Brushes.
Programs: Photoshop 7+, Photoshop Elements 2+, Gimp 2.2.6+
Download the image pack (for those without Photoshop & GIMP)