Apps similar to Rainpaper include rainbowhunt, which is free. Other options are RainEffect. Between them they cover the web and self-hosting.
Relax to the soothing sights and sounds of rain with rainbowhunt's highly configurable, web app. Select from sound effects ranging from light rain to thunder. Or adjust the time of day with Day, Night and a special Stop setting.
RainEffect is an experimental, open-source rain and water drop effect using WebGL. Developer Lucas Bebber has explained in a detailed blog post, the creation of the effect which has since been used for a variety of commercial applications.
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