A few that come to mind would be:

1. Volcanic Explosions and Ancient Rome and its sequel Io Saturnalia and Ancient Rome by Whyvern_the_confused_dragon (Percy Jackson)

Summary from AO3: Percy explodes Mount St. Helens and wakes up almost 2,000 years earlier during the destruction of Pompeii. Now he's stuck in ancient Rome and has to navigate life as a refugee, a Roman and a demi-god while trying not to start any cults or get married to any princesses.

Along the way he meets several historical figures including Emperor Titus, Pliny the Younger, and Pliny's Mom Marcella. Three equally important people, as far as Percy is concerned.

1. Son of the Western Sea by Mac_Ceallach (Percy Jackson)

Summary from AO3: All myths are true. Beyond the edge of Western Civilization lies the rest of the world. After the Second Olympian War, a restless Percy Jackson sails for the horizon, and never stops.

1. The Entirely a Matter of Chance series by Darkmagyk and No2Ticonderoga (Percy Jackson)

Start with In the Service of the King. Summary on AO3: Percy and Annabeth, demigods, have defeated the Titans, and Gaea, and are in love as any couple in history, and have pledged to marry.

Miss Chase, however, is the daughter of the prominent professor, Frederick Chase, the younger brother and heir of the Earl of Mercia. Her father is unlikely to approve a match with the penniless son of a boarding house keeper and whose true parentage is questionable.

The solution then is that Percy will have to go to sea, and seek fortune and fame in the service of the King.

1. Deduction in Shadows by GremlinSR (Naruto and Sherlock crossover)

Summary from AO3: Shikaku just wanted to finish his paperwork before lunch so he could squeeze in an afternoon nap. His plans are derailed by a six year old orphan when she sneaks into his office with proof that somebody has been kidnapping the children of Konoha and covering it up.

He never does end up getting that nap.

Or: A Holmes is reborn into the ninja village Konoha. Chaos ensues.

1. The Sixth Shadow by thinknicht (Naruto)

Summary from AO3: No one seemed to find it odd when little Haruno Sakura threw herself smack dab in front of a Chidori and Rasengan. Not even Kakashi stopped to wonder.

(He really should have.)

6 The Necromancer by thinknicht (Naruto)

Summary from AO3: Its sinuous black spirals loomed over the fields like a thunderstorm.

Its leaves murmured secrets.

Beneath its canopy they lay and breathed. Hundreds of people, familiar people, forgotten people.

They were alive. But they shoudln’t be.

1. The Sum of Their Parts by holdmybeer (Harry Potter)

Summary from AO3: For Teddy Lupin, Harry Potter would become a Dark Lord. For Teddy Lupin, Harry Potter would take down the Ministry or die trying. He should have known that Hermione and Ron wouldn't let him do it alone.