Reader Jessica Basset sent me just about the coolest thing last week. She read Song of Scarabaeus and was inspired to write a role playing game (based on Ars Magica 5th edn) using ideas from the book such as biocyph technology and the Crib and Fringe.
She's written up a player guide and it's wonderful. It includes lots of flavor images along with her unique take on biocyph, infojacks, the Saeth, mash, the leash etc. She's even written some additional history for the eco-rads and the Crib and more, and expanded on that world-building with a detailed caste system (somewhat prescient because one of the books I'm writing now takes place on a world with a strict social hierarchy).
I know some authors are a bit uncomfortable about the idea of readers using their characters in fanfic, and it's a discomfort that goes beyond copyright or other legal issues. (Jessica's game is for her personal use with friends, so she's not profiting financially.) As for me, I'm flattered and thrilled. I love it that people are talking about the characters and places and story that I created. Not only talking about these things, but using them as a springboard for their own creativity. Anyway, you read it here first: bring on the fanfic! (I think here I'm supposed to say "For legal reasons I can't admit to reading it.")
I'm not a gamer but MCP is, and is always threatening to write a sci-fi RPG set in a world much like the one I wrote, so that I'll have no option but to play it. For now, I'm just happy that someone out there has enjoyed my book and incorporated some of its ideas and "essence", as Jessica puts it, into her own creative outlet.