Kkat's Fallout: Equestria

This is arguably the biggest fanfic in the fandom. It's spawned over 250 side stories, translations into multiple languages, and countless pieces of fan art and fan music, and was considered notable enough to be mentioned in Fallout's Wikipedia entry. You can check out all the fan content at the Fallout: Equestria Resource. Marshmallow ponies and post-apocalyptic wasteland... who'd have thought?

[Adventure][Dark][Crossover] • 607,200 words
Fallout. With ponies! Set in an alternate future, one pony must learn to survive in a blasted, poisoned land... and possibly, with the aid of friends made along the way, bring new light into the darkness of post-apocalyptic Equestria.

Hit the break for an interview with Kkat (of which there have been quite a few), and links to Fo:E out on the ponynet. And of course you can grab ebooks suitable for your terminal or PipBuck over on the Downloads page!