Same universe, different series

Yet again, the WriYe blogging circle provides me with a subject that makes me go blank. Aside from a brief reference to a scary-beyond-all-reason girl called Stella, I've never really re-used a universe. I don't even know if that counts - it's just a convention of my writerly multiverse that girls called Stella are always bad news. She's appeared in three separate stories, most recently as the personification of my Inner Critic.

So, let's talk about other people who've re-used a universe.