A story which heavily features letter-writing (or something similar) between the protagonists.
Post-mort (Harry Potter) by Duj
A letter, deliberately delayed, reaches Hermione the morning after her N.E.W.T.s.
The Ethernet Chronicles (Harry Potter) by Pythia Delphi
Post Vold-War II, the Wizarding World has adopted a new-fangled notion: blogging!
Neither the Laurel Nor the Rose (Harry Potter) by Darned Child
The Malfoy family expects to receive the finest service possible, no matter what the task. When Draco seeks the affections of Hermione Granger, he enlists the aid of a careful wordsmith - one Severus Snape.
Proestigium (Harry Potter) by Lady Strange
An epistolary story based on the WIKTT 'Arsenius Jigger' challenge. There will be 42 letters in all. Correspondences trace the changing lives and times of the various characters in the HP universe as well as some new characters.
A Stranger's Kindness (Harry Potter) by Bohemian Rhapsodi
Hermione Granger always knew that her life was on hold for Harry's cause, but a gesture of kindness from a mysterious stranger will soon begin to change her mind. AU, HGSS, In response to a 'Daddy Long-Legs' WIKTT challenge.
To Be Lived (Harry Potter) by Lady Birdington
Anonymous letters from his savior leave Severus unsettled.