In Game of Thrones (ahem, and A Song of Ice and Fire), a big question is: Who is Jon Snow's mother? But a better question is the broadest version: Who are Jon Snow's parents? We know he was raised as the bastard son of Ned Stark and an unnamed woman, but this fan theory dares to ask...what if Ned isn't his biological father? Take a look.
(Note that the video contains major spoilers for Book 1/Season 1, and minor spoilers for Book 2/Seasons 2-4. If you've read or seen the first book/first season, you're safe watching this.)
If you're not into the "let's watch a six-minute video" thing, the theory is pretty well summarized on A Wiki of Ice and Fire, minus some extra context, family trees, and direct quotes from the books.