One of the most prominent and well-known venues for fan-generated content in both the furry and brony fandoms is fan fiction.  These are fan-written stories which create 'what-if' type scenarios based around a core cannon of story lines.

DRWolf Narrator by DavionXY:
http://davionx.deviantart.com/art/DrWolf-Narrator-377863090

TericDragon Avatar Drawn by Teric:
http://www.furaffinity.net/user/teric/

"Friendship is Magic: The Night the Magic Died":
http://www.furaffinity.net/journal/3722079/

"Harry Potter and the Methods of Rationality":
http://hpmor.com/

Various Art Pieces used:
http://pinkie.ponychan.net/chan/files/src/134677301510.png
http://24.media.tumblr.com/6c0293c7f419ebe71c83069e6c81cc8e/tumblr_mph6r5yRH61rxmai6o1_500.png
http://fc00.deviantart.net/fs70/i/2012/110/5/7/equestria__s_first_and_last_human_by_ceehoff-d4wzxsl.jpg
http://yerf.metafur.org/doovbern/superpan.jpg
http://fc00.deviantart.net/fs70/i/2013/172/1/c/equestria_girls_mane6_by_asdf314159265-d6a0f2z.png
http://fc08.deviantart.net/fs71/f/2011/152/b/3/kung_fu_panda___iron_lady_by_osato_kun-d3hri28.png
https://fc02.deviantart.net/fs71/i/2011/154/6/f/mlp__star_trek_by_cartoonlion-d3hz60h.png
http://fc05.deviantart.net/fs71/i/2012/020/e/5/harry_potter_and_the_methods_of_rationality_cover_by_mike_obee_lay-d4n2119.png
http://hpmor.com/wordpress/wp-content/uploads/2013/06/UnknownCartoonsLover_3202049.jpg?75195f
http://26.media.tumblr.com/tumblr_ljeyrmPJ0F1qikm5io1_500.png
https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcQhGI7DcCKNKse9Dqsf4nusQmS4xs78Zky-B41S3nxwmzekaBLdLg
http://raccoons-bookshelf.net/Site/RBS-Kacey.jpg
http://d.facdn.net/art/tavimunk/1310963927.tavimunk_fox_writing-01.jpg
http://static.fjcdn.com/pictures/A_392d98_202521.jpg

My Little Pony Characters and Images belong to Hasbro.