While watching one of Gavin Hoey's YouTube videos (looking for holiday light info), I came across this one. Towards the end, he added a smoke machine, took the photo, then in post he filled in snow with a free Photoshop snow action. He links the action in his comments. If you forward the video close to the end, you will see how the action adds other layers and how he edits the image once the action is finished.
