Adobe Spark (https://spark.adobe.com/about/) - Turn your ideas into impactful social graphics, web stories and animated videos—in minutes.
Atavist (https://atavist.com/) - Make a story, design it your way, share it. A powerful tool for media-rich stories.
Storybuilder (http://storybuilder.jumpstart.ge/en) - Build and Share the Next Generation Story. StoryBuilder allows anyone with text, pictures and/or videos to combine this content into a creative story that can be published and shared with the world.
There is very high enthusiasm around multimedia stories and tools are popping up on and on ... but some are already closing down as they don´t fullfilled what they promised or havn´t been successfull such as "Storehouse" (closed July 2015, old stories are shifted to an archive)