Cosmic Powers Fan Fiction: Fan fiction starring Silver Surfer, Adam Warlock, Quasar, Galactus, Thanos, and other cosmic characters of the Marvel Universe.
eX -- An Online Fan Fiction Novel: An online novel, posted in episodic form, featuring the Marvel Comics characters called the X-Men.
Firebird's Marvel Fanfiction: Marvel Fanfiction by various authors.
Fonts of Wisdom: Kitty Pryde and Pete Wisdom (Excalibur) fan fiction.
Generation X Fan Fiction: Contains Generation X fan fiction written by Francisco Charrua.
Maria's X-Men Fan Fiction: A collection of fan fiction stories featuring Marvel Comics' X-Men.
the arcHive: Fan fiction and art.
The Great Comic Book MiSTing Site: A humorous collection of MSTings (MST3K-style "revisions") which pokes fun at bad comic-book fan fiction and bad comic books, too.
Ultraverse: A New Twist: Fan fiction based on the Malibu Comics characters in Marvel's Ultraverse.
X-Genesis: A revisionist view of Ultimate X-Men. Fan fiction stories starting the X-Men legend from scratch.