Colonel Robert Gordman finds himself trapped in the land of Equestria after being mysteriously transported from his home in Dayton Ohio to the Froggy Bottom bog. Loneliness and Starvation over come him, until he discovers the bizzare inhabitants of this new land. He has a son back on earth and his wife is expecting another kid, Robert cannot afford to stay here any longer. He must find a way to return home, even if it means kidnapping ponies in order to gain the attention of Celestia.

(Attention: Permanently Cancelled) The UNSC Hastings crashes into Equestria's moon after a slip-space incident. The survivors make it to a pelican and crash-land in the Everfree forest. Their arrival throws Equestria into turmoil as an ancient cult plots its return. On top of that, a disillusioned officer in the Equestria military plays her hand.

Robert Parker is a 24 year old ex-web developer living in New York city. In the wake of the conversion movement, he has seemingly lost everything. Haunted by the same nightmare every night, he soon realizes that he must make a choice.

To those unfamiliar with the Conversion Bureau: it is a scenario created by an author named blaze in which Ponies either appear in the human world or have lived beside humans without incident for millennia. One day, in the face of all the damage humanity has done to its world, the ponies offer a solution: undergo conversion to become a pony and allow the world to be inundated with Equestria's influence: thus healing it and bring forth a golden age.

Many different authors have taken many different approaches to this idea, all of them are worth checking out.

http://www.ponychan.net/chan/fic/res/53616.html (The current thread)
http://www.equestriadaily.com/2011/04/conversion-bureau-compilation.html (The compilation post on EQDaily. *NOT UPDATED*)

The Metro tunnels of Stalliongrad shelter the last remnants of ponykind from the ravages of the Apocalypse. Beneath the earth, ponies struggle to survive against mutants, radiation, and each other. But when an unstoppable threat emerges from the darkness of the Metro, one pony must brave the shadows to save not just his home, but the very existence of his people.

What if Twilight and Trixie were both zeppelins who were in love? At last this age-old question has been answered!