Discworld

Mostly Human by Donna Immaculata
Lord Vetinari challenges Death, whose position is being filled temporarily by Susan Sto Helit. Captures the tone and rhythm of Pratchett's writing, with just the kinds of twists, turns and references that fanfic was created for.

In a Name by Afrai
This character piece opens with an exchange between Havelock Vetinari and Sam Vimes at John Keel's funeral, and goes on from there. Quiet, but subtle, appropriate for anything from the Patrician's POV.

Model Manners by Mistress Scarlett
This is hilarious. Pure and simple. Carrot and Angua's daughter joins the Watch, and amid the ensuing chaos, something is affecting the minds and bodies of Ankh-Morkpork's girls. Great dialogue, comedy in keeping with the spirit of the series, and young Lord Sammy Vimes is kinda hot. More importantly, Del is a likable and plausible OC.

Say Yes by Mercator
A seamstress takes on the Patrician, and wins. Sort of. It shouldn't work, but it does. The idea (and character) isn't necessarily sustainable over a series - I only skimmed the sequels - but this battle of wills and sex is a nice way to spend a cosy evening.

Defender of the Crown by Sam Vimes
Vimes reaps the karmic reward for his lifetime of staunch republicanism when Pseudopolis finds itself in desperate need of a monarch, and Sam is just the man for the job. FF.net does unspeakable things to formatting, but this is that rare beast, a story that rewards the effort. Great jokes, great characterisation, and some great comedic and dramatic set-ups.

In the Blood by Sam Vimes
A close encounter with a werewolf in the climactic chapters of The Fifth Elephant finally catches up with Sam.