The Ninja of Unlimited Blades by chimmon
Shirou was saved from the fire that consumed his village by a young ninja called Haku. Now, he seeks to become a useful tool to Zabuza, just like she is.
Crossover - Naruto & Fate/stay night - Rated: T - English - Adventure - Chapters: 9 - Words: 99,243 - Reviews: 188 - Favs: 229 - Follows: 281 - Updated: May 27 - Published: Feb 15
Despite it's terrible sounding premise this fic is actually written by a competent author who handles the story fairly well. There isn't much in the way of fannon and the characters actually seem to be fairly believable.
By the same author is
Harry Potter and the Counter Guardian By:
chimmon
Before his death at the Department of Mysteries, Sirius had sent Harry a book to help him pass his time at 4 Privet Drive. This will result in Harry Potter inadvertently summoning a sarcastic wizard in red robes who calls himself Archer. How will this Heroic Spirit change the world?
Rated:
Fiction T - English - Fantasy/Adventure - Harry P., Archer - Chapters: 12 - Words: 80,555 - Reviews:
152 - Favs: 475 - Follows: 370 - Updated: Jul 7, 2014 - Published: Apr 21, 2014 - Status: Complete - id: 10287211
Also a decent story despite how the two settings seem like they'd be terrible things to mix. Not as good as the other fic in my opinion but probably one of the very rare readable HP/FSN fics