Desktop Snow for Windows makes it snow on your desktop, behind the windows you're working in. Configure snowflake type/number/speed, configure wind direction/characteristic, whether it's a blizzard or flurries and whether the snow settles or not.

There is no timeout, and there are no restrictions apart from a small nag-screen that displays when the program loads and exits but not when it's running. Makes your desktop very wintery, christmassy and generally jolly and festive.