How would things be different if Twilight had never made it to Ponyville? Twilight Sparkle finds out just how different things could be when she is forced to travel to her own past while chasing down a mystery; a plot to erase Princess Twilight Sparkle from history.

Landing in a time just weeks after the one thousandth Summer Sun Celebration, she finds a world shrouded in an eternal night, where the Elements of Harmony were never recovered. Can she fix the things that should have, but didn't happen? Or is this Equestria destined to head down a different path?

Chinese Translation by Hell Organist

Shawn Spencer, psychic detective, is a thick-tufted boy genius who ice skates through life on polished blades of snarky eloquence. He has tested himself against some of the greatest criminal minds of his time and emerged victorious... though often through unbelievable luck and not a small amount of bumbling.

Now, he faces a challenge unlike any before. One that will test his mind, his sanity, and his adaptability as he is thrust into the midst of an unfamiliar world and the ponies who populate it.

The Mane 6 face their own challenge, as an old enemy, tempered in the crimes of a much harsher world than theirs, returns to Equestria. It is a challenge they are ill-prepared for and one that only their strange, new, alien friend can help them overcome.

When the Cutie Mark Crusaders dig up an ancient magical artifact, they unleash a spell on the town that allows everypony to see a "lie meter" floating above each other's heads. While attempting to solve the mystery, Twilight Sparkle has to analyze the tenuous balance between friendship and honesty. She doesn't like what she finds.

Based on a story idea conceived by theworstwriter
Written and posted with his permission

Once upon a time, in the magical land of Equestria...

One week ago, Twilight Sparkle was crowned the fourth Princess of Equestria. Tonight, Ponyville hosts the 1003rd Summer Sun Celebration, and Princess Celestia and Princess Luna want a vacation. With two fine young princesses like Cadance and Twilight to take over for them, nothing could possibly go wrong.

(Title image hacked together from vectors by sakatagintoki117 and Chrome-MMVII.)

After a mishap involving the Transportalponder, the Courier finds himself in a place he doesn't recognize. With no way back and nothing else to do, he decides that wandering into the nearest town is the best option. Being the Courier, things aren't that simple...

A Fallout: New Vegas/My Little Pony: Friendship is Magic crossover

The style alternates between omnipresent third-person and the Courier recounting his tale to... Well, take a guess.

A normal day was all it took.
A simple failed prototype, a missed experiment.
There was an explosion, and I woke up. I don't really know what happened between those two events, but I'm fairly certain I shouldn't have awoken in a small, dark box.

I'm scared.

Cover art by the amazing Avatar of Madness!

This is a fic based on the Time Loop Files, the format of which was first codified by Innortal on fanfiction.net.
Here's a brief guide.
The basic idea is, one character (the Anchor, usually the main character of a fictional setting) is looping back to the very first moments of their parent series every time either they die or some kind of time limit expires. They of course end up monumentally stir crazy given enough time.
Eventually others start to loop as well.
The reset is not always perfect. Sometimes a loop's history will be different to the "prime" loop, or canonical plot.
"Crossover" or "Fusion" loops also occur, randomly. These can involve the home loopers having a guest, or the anchor for one universe spending time in another, or replacements of one character by another.
Vacation Loops are where the Anchor (or others) decide/s "buck it" and lets off steam by doing whatever comes to mind. There's little or no attempt to maintain the original timeline.

In addition, and particularly more recently, it should be known that this story has a collaborative element to it - people may submit loop ideas or loops, ideally at the Spacebattles thread (a link to the current version of which is on the Trope page), and they may become included.

There's two page image options, thanks to Filraen - the current page image has been edited by the original creator from this work of theirs.. The other one is here:
http://hourglass-vectors.deviantart.com/art/MLP-Loops-466030169