Two of them!
The first is Appledashery. It's a bit of an odd duck. Short chapters but updated daily. The story so far is pretty good though, and I've spoken with the author - it'll last a good long while yet. I can't wait to see where it goes. This plays mostly from Rainbow's POV, it seems. And it deals with her adventures in crushes and romance, and all the stuff that goes with it. And a bit more on top of that. It's almost criminally underviewed, so here's a bump for people to read it :P
Oh, and if you hadn't guessed, it's shippy, and has Appledash. Shocking, I know.
The second is: The Mailbox Compilation!
What started as a collab project in a dedicated fimfic group, with six authors each impersonating one of the mane 6, eventually turned into a lengthy set of stories and back and forth letters between our favorite ponies. The letters have now been compiled into a fic, the first chapter of which you will find here: The Mailbox Compilation