The Long Con, by lightgetsin. This is a funny and charming story set in Lois McMaster Bujold's Vorkosiverse, and the characters and voices are perfect and wonderful. If you know the fandom - well, you've probably read it already.
My own story was Maggie's Kitchen, which is set in Laurell K. Hamilton's Anita Blake universe (*before* Obsidian Butterfly). This was a lot of fun to write - it gave me a chance to write an old-fashioned murder mystery, which I have never really done before.