A ten page short:
What if all the magic we see in the show is exactly what it appears to be? Does that concept scare you? It should.
After a shocking discovery changes his life, a young griffon follows in the hoofprints of his childhood hero, and mounts an expedition deep into the frozen west.

Both 'The Brightest Shine' and 'Piracy' redirect here, and with good reason; read on and it will make sense.

Sometimes, when you've lost so much, when everything you do seems to come out wrong, and when you don't feel like anything but a failure, it can seem like there is no other option but to give up.

But when one pony feels so lost, so inconsolable, so forlorn, that she makes a life-changing decision that almost leaves her shattered forever, it is up to her friends to bring her back.

When you lose something that you can't replace, when you love someone but it goes to waste, and when the tears come streaming down your face, is there anyone who you can turn to...

...to fix you?

When Applejack and Rainbow Dash wake up, hungover and in the same bed, they've only one question: how can we get even? A day spent tormenting their friends by turning one of their hopes on its ear pays off, but not quite in the way anypony expected.

With the time she has left, Rainbow Dash seeks what is truly important in life before moving on.

"My name is Lyra Heartstrings, and you will not remember me. You won't even remember this conversation. Just like with everypony else I've ever met, everything I do or say will be forgotten. Every letter I've written will appear blank; every piece of evidence I've left behind will end up missing. I'm stuck here in Ponyville because of the same curse that has made me so forgettable. Still, that doesn't stop me from doing the one thing that I love: making music. If my melodies find their way into your heart, then there is still hope for me. If I can't prove that I exist, I can at least prove that my love for each and every one of you exists. Please, listen to my story, my symphony, for it is me."

Cover pic by Spotlight: askspotlight.tumblr.com

Twilight isn't having a very good day. An experimental spell blew up in her face, an army of changelings is attacking Canterlot, and she just died. Yet somehow, it looks like it's going to keep going downhill from here.

Given the chance to correct what's gone wrong, Twilight swears she's going to fix all this even if it kills her. Which it will. Frequently.

Cover art by Pixel Prism
Spanish Translation
Series TV Tropes page

After the Changeling invasion, Celestia and Luna make the decision to have Twilight investigate the Elements of Harmony and solve the mystery of their power.

A thousand years later, Rainbow Dash awakens in a world she does not recognize. She has no idea how or why she came to be there. She does not know the language, the geography or any of the rules of this new time. She soon finds allies in Star Fall, a scholarly pegasus whose Talent is Magic, and Astrid, Star Fall's Griffin guardian. Together they set out to discover why Dash has been sent to the future, and how she might return to when she belongs.

War threatens the nations, a Nightmare stalks the shadows, and Rainbow Dash's arrival has turned her into a wild card in a deadly game that pits the law on both sides of a broken Equestria against a mysterious criminal figure, Max Cash. Yet the stakes are higher than anyone knows, for in her research Twilight Sparkle discovered a dark secret about the Elements, something that will shake the foundations of the world.

Cover image by SierraEx
Has a TvTropes page

When a freak reading accident causes Twilight to be admitted to the Ponyville hospital, Rainbow Dash is right by her side. Compelled by a sense of duty and repayment, she takes it upon herself to cheer up her friend.

Twilight Sparkle, becoming anxious at the upcoming Summer Sun Celebration, which she believes marks the return of Nightmare Moon, beseeches Princess Celestia to do something about it.

Celestia, however, has another, more important task for her; making sure that the Summer Sun Celebration goes off without a hitch. Every year, a different town is chosen to receive the honor of hosting the grand festival.

And the lucky city chosen this year, for this fateful holiday, is none other than the city of Manehattan...

Cover image by Conner Cogwork.

Rainbow Dash struggles to figure out why Pinkie Pie keeps acting weird, and seems so happy about spending Hearts and Hooves with hanging out with her, while also puzzling over the fact that Cloud Kicker and Blossomforth seem to be going out of their way to help the two of them enjoy the day.

Cloud Kicker and Blossomforth need to have their own tags, because they're awesome characters who deserve to have more stories written about them.

Has a semi-sequel-follow-up-side-story:"The Life and Times of a Winning Pony."