Dungeons & Dragons
Like most people, I could tell stories when I was wee little. Unfortunately most people I know either lost or failed to level up their story-telling abilities during middle and high school. I inadvertently maintained and grew my skills through Dungeons & Dragons.
It wasn't until much later that I realized what a gift those years of imagination were. D&D may seem like an unconventional route to becoming an author, but it played a pivotal role in fueling my creativity and raising my understanding of what makes a good story.
Many famous story tellers loved D&D. To name a few, we have Joss Whedon (TV & movie writer), David Lindsay-Abaire (play write), Junot Diaz (2008 Pulitzer Prize winner), Cory Doctorow (Sci-fi), and Sherman Alexie (memoirs).