Letters. Everyone writes them, and they're not just for ponies. Be they carriers of serious news, friendly correspondence between pen pals, or declarations of heart and heartbreak, each one is written with a clear purpose or goal in mind.

Celestia uses them to try and keep her faithful student and friends in line, and try to keep an unbalanced world on the straight and narrow.

This is much harder than it sounds.

Now with an audio reading by Lady Tenkage!

Side Stories:

Old-Fashioned Bar Hoppers, featuring Celestia, Mayor Mare, and Granny Smith shenanigans.
Journal of an Adolescent Clone, featuring Sunny and Moony shenanigans since Season 5.

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