The Boris Continuum Complete Snow filter for Apple Final Cut Pro and Motion is free until March 1st. It's an auto-animated snow generator with extensive options that allow you to create drifts or make flakes pile up along the edges of an alpha channel.
Download the free Snow filter here, and look at the tutorial too. You must install the free BCC Fxplug before March 1, 2008.
There's also a free "blizzard" effect from DVCReators.net.
Update: for other options, see Creating snow in After Effects.