A few days ago, I posted a book I made---Snowflakes Everywhere! This book has been very popular with my kids. I have since found an app to go along with it. It's called:

Pocket Snow Storm! A Virtual Reality Blizzard! (White Christmas Edition!)

This app is easy. You open the app, look at something with the iPad, and tap the screen to make desired snowstorm levels. You can also have the camera point at the iPad holder---some kids really like looking at themselves and are intrigued to see snow falling down on them. Check out the YouTube video for a demo.
The picture taking feature in the app costs 99 cents--I felt I could afford that (the cost of a half cup of coffee). I think I'll be using this all month. It's a very nice tool for joint referencing, engagement and fun. See some potential uses below.

The results and uses:

Even the most disengaged kids loved this.
You can use it for following very simple directions (i.e. "Look at Ms. O!" as the child is looking at the iPad screen viewing the snow falling on things and people in the classroom.)
Kids seem to sustain attention to screens, where they don't in real life, even if the screen is mostly real life. I had one kid make comments on all of the things he was seeing in the iPad screen (when it actually was in his everyday world). We had a conversation which doesn't happen often with this child!
For the slightly more advanced students, you can have kids help make their own 'Snowflakes Everywhere' book. See the sample pictures below---simply have the kids point and shoot at their favorite things, print the pictures, make a book, have the child dictate what he sees (or he writes or types the sentences).