Twilight and her five friends are transported during a botched spell, across dimensions to... Ponyville, Equestria? But not the same one that they knew. Sitting on a bench is Harpsy and Babar. The Doctoress avoids apples from an aspiring Applebuck. And in a library across town, six stallions join their forces as they try to decipher the appearance of filly look-alikes of themselves around Ponyville!

After being banished by her sister, Luna is frustrated with her fate. It's a shame that there's no one on the moon she has to take her anger out on...

... Or is there?

After Nightmare Night, Luna invites Twilight to tea, and they start spending more time together. Princess Celestia teases Luna about it sometimes.

A letter from Celestia interrupts one of Pinkie's parties with disturbing news. Now friendships will be pushed to the limit as Twilight struggles to uncover which of her friends is not who, or even what, she appears to be.

(While not a romance, this does contain a shipped pair)

in Russian

All of Equestria is united in celebration for the return of their beloved Princess Nightmare Moon. But when Celestia and everypony else recieve her she's skeletally starved, filthy, and very nearly dead. Meanwhile, while everypony's eye is turned, a lusty Blueblood pursues Twilight Sparkle, intent on adding her bloodline to his own so his family might step closer to godhood. (Alternate Universe, there never was a Princess Luna)

Princess Luna decides that the Sisterhooves Social is the perfect opportunity to spend some quality bonding time with Princess Celestia. Do Sweetie Belle and Applebloom stand a chance or do the princesses of Equestria have this competition in the bag?

Twilight Sparkle is called to Canterlot Castle where something has happened to Princess Luna that has reverted her to a weakened state, she has to live with Twilight whilst she recuperates her magic.

Suppose you found yourself in a strange body and on an unknown planet, with only a lonely dentist for company. What are you willing to do to get home? How hard would you fight? Does it matter if you're a princess or just a person?

Don't expect quality from this fic.

Being a creator means creating your own world, where you pour your love and heart into it.

In a stormy night, after a strange event in her office, Lauren Faust gets teleported into the magical land of Equestria. The land... that she created. What will she do now? How will she return to the reality from her own imagination? Will she even be willing to leave?

It is the year 2038, and the Earth is slowly being consumed by a space-borne monstrosity dubbed the Black Tide, which is using nanotechnology to remake the planet into something hideously alien.

Erin Olsen works for Project Harmonics, humanity's last-ditch effort to find a new world before the Tide can wipe them out. But when that world is found, and it turns out to be occupied, Erin will need to find the courage to face the unknown in order to save the inhabitants of both worlds.

(Many thanks to Easteu for the fantastic cover image for this story!)

Now with its own TV Tropes page

There is now a follow-up to this story: Sunflower - Side Projects
Sequel: Project Sunflower: Harmony