http://www.rainymood.com/The sound is very nice for relaxation; it is rather calming. I've been playing it at low volume every night for the last few nights...I have been falling asleep more quickly and awakening more pleasantly.
PROTIP: If you are thinking about the sound/focusing on it while you're falling asleep the rain may be incorporated in your dreams, i.e. it will rain in your dream. You can teach yourself to use rain as a dream clue to recognize a dream state and initiate a lucid dream.