This is simple as3 class. You can add "MouseObject.as" to your "Classes" dir and use "main.as" in your project.
You can change "sneg" object in library to any other image for snowfall.
Setting MouseObject.as:
private var min:Number = 1;//min count of objects
private var max:Number = 5;//max count of objects
private var minCoord:Number = 0;//min coordinates
private var maxCoord:Number = 15;//max coordinates
private var speedY:Number = 4;//speed of snowfall
private var alfa:Number = 0.02;//speed of fadeout
Setting Main.as:
private var density:Number = 1;//time interval of creating objects