Hmmm... Fan Fiction Christmas Stories...
All time favorites:
NGE 'What Every Girl Wants' by Random1377 - [Status=Single File] - [Last Dld=01/29/2003]
TM 'I'll Be at the Masaki Home For Christmas' by Jeff Morris - [Status=Single File] - [Last Dld=12/23/2004]
Some others I've read:
B5 'B-5 Christmas' by Jon Benson - [Status=Single File] - [Last Dld=11/21/2002]
EL 'Christmas Tree' by Crystal Remnant - [Status=Single File] - [Last Dld=03/18/2006]
NGE 'A Very Messy Christmas Miracle' by dennisud - [Status=Single File] - [Last Dld=12/26/2003]
NGE 'Forgotten Christmas' by Advantage - [Status=Single File] - [Last Dld=12/26/2003]
NGE 'Mistletoe' by Hououza - [Status=Single File] - [Last Dld=04/01/2005]
NGE 'Under the Mistletoe' by Random1377 - [Status=Single File] - [Last Dld=01/29/2003]
RANMA 'Underneath The Mistletoe' by WING Higley - [Status=Single File] - [Last Dld=10/01/2004]
RANMA 'Well After Christmas' by Siaru - [Status=Single File] - [Last Dld=10/08/2003]
R-AA 'Christmas Addventure Spamfic' by Kender - [Status=Single File] - [Last Dld=12/27/2004]
VD 'Once upon a Christmas Eve' by ktx - [Status=Not Complete] - [Last Dld=06/27/2006]
Also, I'm not listing it, since it has lime content, there was a Ranma Betfic by Metroanime that was called 'Christmas Cake'