Well, with the newest three, at least:
Tales From Beyond the Crypt
Remember all those dead minor/major characters? They get to go to other dimensions when they die! Good for them... [Spoilers through Chosen and You're Welcome. Also individual spoilers for different 'verses, noted in the header of each chapter]
Multiple Crossings > Miscellaneous -:- TeamSocket -:- 15 -:- Chapters  -:- Published [2004-02-05] -:- Updated [2004-02-05] -:- Reviews 
Takes place during Christmas of OotP. What's to say that Percy didn't go visit his father? OotP spoilers, of course.
Non-Crossovers & Non-BtVS Crossovers > Harry Potter Fanfic-Other -:- TeamSocket -:- U -:- Chapters  -:- Published [2003-12-24] -:- Updated [2003-12-24] -:- Reviews 
Good Luck Lies in Odd Numbers
And they don't get odder than this. Based on the premise of "What if Jonathan was a mutant?" S7 and X2 spoilers.
X-Men > Other- General -:- TeamSocket -:- 15 -:- Chapters  -:- Published [2003-12-24] -:- Updated [2003-12-24] -:- Reviews 
Oh, and y'all need to go read them, too.