The Case of the Unwelcome Owl by pandarus (on DW) is a short fic (7458 words), it is also Gen.
I like it for the same reason I like my own Harry Potter crossover - it's BELIEVABLE. And what I mean by that is that the universes are blended in a very believable way - not only that, but the things about the established characters that the author has to tweak are actually tweaked in a way that is completely in keeping with the characters.
Also, it is amusing.
There's only one slight canonical mistake in it (an owl delivering a personal letter would not have to be paid - only newspaper delivering owls require payment)...so, overall, I'd say it's pretty damn good.
It also leaves things way open for delightful sequels, though, if the author doesn't proceed with caution, she could completely ruin why I like the story - but isn't that the case with everything?
So, yes, this rec is for the few of you who watch Sherlock and like crossovers. I know there's a good chunk of people who friended me based on their love of crossovers, so I doubt that bits the problem is you AREN'T clicking on the link. So, I'll add that if you haven't watched Sherlock, you totally should - because it is great and has so much goddamn potential for everything that it KILLS me. I want to turn that show into a lake and swim in it.