Less of a one-shot and more of a side trek, but it's a pretty solid mystery. Would absorb about an hour or two of time at your table, and has some good NPC interactions. Overall, for 99 cents, you could do much worse!

I'm not sure I would even qualify this as a one-shot. It begins with a fight that the PCs watch but could be involved in if they want and then the subsequent investigation of why the fight occurred. That's it. It establishes a potential rival to the party but that's up to the DM.