I need a simple shader to be used on planets which allows planets to cast shadows on their moons and vice-versa. Can anyone suggest one.

P.S. I tried to use the shadow shader from the download section but it did not work well. Also the atmosphere shader from the wiki section made some strange shadow artifacts

Have you tried the built in stencil shadows? They don't require a shader (but look better with a blur shader that smooths the sharp edges).