A lot of my story ideas stem from other stories, whether in fanfic, book, or movie form, but my favorite "what inspired the idea" story is for Song for the Undead, which started as a joke.
Rose, Dee, and I were having a group chat on Yahoo Messenger late at night, as we did most nights back in the summer of 2008, and the chat had gone quiet, so Dee asked, "Is everyone dead?" and I jokingly replied, "Yes." She said, "Oh noes! Julie is a ghost," and I said, "I prefer zombie," which got us bantering about zombies. Rose was still quiet through all of this, so I (again jokingly) said, "Hey Rose... Backstreet Boys, zombies, collab? *hopes to summon Rose back*" because Rose is such a collab whore. And whaddya know... after some time, Rose came back, and we started talking (still jokingly, I thought) about a Backstreet Boys zombie story, in which the Boys survive the zombie apocalypse and fight the undead. We were just joking around at first, and then it got to the point where we were like, "Are we seriously going to write this? Well, why not? It sounds cool; we should!" So we did! I think people who stumble onto the story nowadays probably assume it was inspired by The Walking Dead, but this was years before that was a show (it was a comic, but we'd never heard of it until the show) and before the current zombie trend - back then, who knew?! I've learned through collaborating that some of the best ideas start as jokes LOL.