I've heard recently about Rainymood, a website where you can add Rain sounds to tracks on youtube.
And it's true, rain really makes songs better

Rainymood.com
Youtube
You can combine a song of youtube with rainymood's website just by pasting the end of the youtube link with rainymood's website's adress.
Post the best ones you've tried

Here's my first one :
http://www.rainymood.com/watch?v=gEsUiQIyzd8