Snow is a project mainly written in JavaScript, it's free.
snow
Javascript-based snow.
You'll need:
Add a div to contain your snow:
Create a snow field. Snow fields need a width, a height, and (optionally) a renderer: var snowField = new SnowField("my_snow", { width: 750, height: 400, renderer: 'canvas' });
Add some snow: snowField.addFlakes(350);
You can stop and start the snow: snowField.stop(); snowField.start();
Snow has a flexible rendering architecture, and includes renderers for canvas, CSS and SVG.