[Harry Potter] Harry Potter and the Prince of Slytherin

This is a rather enjoyable (and long) fanfiction for Harry Potter. It's set in a somewhat similar world to canon, but is modified in various ways. Such as Peter Pettigrew not hiding out as a rat.
It is a Wrong Boy Who Lived story (this being a story where Harry has a sibling which is thought to be the Boy Who Lived), and it has loads of other common tropes but it applies various twists upon them that makes it a lovely experience.