jQuery Snow
Download here.
Introduction
This extension decorates your webpage with falling snow. To use this extension, call the plugin on $(document): 
    $(document).snow({ SnowImage: "snow.gif" });
The "snow.gif" file is provided in the archive. Arguments:
SnowImage:
The URL of the image-file (required).
Quantity:
The number of snow flakes to show.
HideSnowTime:
Hide the snowflakes after this number of seconds.
ZIndexStart:
Each snowflake will be assigned z-indexes starting with this (default is 0).