Snow effect experiment in Actionscript 3, with random wind ( I used two sine waves actually). The SnowEffect class could be changed to different kinds of snow effects, as its wind, size, horizon height, intensity are all ready to be updated. Check labs.neatfilm.com to see the effects.
Source code (BSD License) available to download: Snow.zip
Update: Nov 13, 2007. Update for Flash CS3, last version was created in Jan 2007 with Flash 9 before Flash CS3 release.
Update Apr 2011: It seems some ppl still look into this years-old experiment, I made a quick update for necessary changes. The Flash file now in CS4 format(minimum version CS5 can support).
Math graph behind the snow effect as followed.