Possibly the best fan-fiction, more enjoyable than a lot of professional work, is the novel series for Thief. It is even written in a fashion so that anyone unfamiliar with the game itself could enjoy it just as well. If you like the Thief universe, this is a thrill. It's called Correspondence of Thieves (rewrite) http://cosas.ttlg.com/cot.asp
Also, one that is pretty much equally wonderful, is the novel based on the story of Grim Fandango, which pretty much tells the whole thing in a novel-like manner, without weird puzzles and all that, and moreover it fills up every one of the gaps that were in the game. It really is as if the game was based on this work, instead of the other way around. http://www.grim-fandango.com/novel.php
Actually, the Grim Fandango novel is a must-read for pretty much everyone, because it is also of a very reasonable length, whereas the Correspondence of Thieves novels amount to several thousands of pages, I believe.
Both of these works are downloadable in PDF format, completely free of charge of course.
I just feel like I HAVE to recommend them.
Anyway, I generally avoid fan-fiction, unless it's about something I really love and which has no more professional additions story-wise. Also if it's a very big and interesting project.