I've looked at a few apps that allow you to layer effects over videos and found them impressive. Today I'm looking at Green Screen Movie FX Studio (US$4.99, universal) for iPhone, iPod touch and iPad. It goes a step beyond some others and lets you create impressive green screen effects. The app also allows tweening so you can set a few positions for an effect, like a tank, a rocket or dinosaur, and it fills in the animation between the starting and ending points.
The app features 88 video FX layers, which you can resize, rotate or move along a timeline. Effects include explosions, fires, weather (snow, fog, hurricanes), space ships and much more. In addition, there are backgrounds you can insert behind people in your videos like Niagara Falls, Paris, beaches, etc.