We're gathering links to compile a Yuletide edition - if you've found a crossover fic while archive-diving through this year's
Yuletide Treasure, please drop a link here, so we can collate them all into one post.
ETA: Still updating this list as of 31/12/08, so feel free to leave links.
A List of Fandoms Represented
Alice in Wonderland
Amelia Peabody
As You Like It
Beautiful People (UK)
Beauty and the Beast (Disney)
Blue Beetle
Boston Legal
Brotherhood of the Wolf
Buffy the Vampire Slayer
Burn Notice
Chrestomanci
The Chronicles of Narnia
Classical Mythology
Constantine
The Dark is Rising
The Dark Knight
Die Anstalt
Discworld
Disney Princesses
Doctor Who
A Dog's Breakfast
Dresden Files (TV)
FAKE
Fringe
Harry Potter
Halloween
Hamlet
High School Musical
House of Leaves
House MD
How I Met Your Mother
Jake 2.0
Incredible Hulk
Indiana Jones
Jeeves and Wooster
Journeyman
The Little Mermaid (Disney)
A Little Princess (Frances Hodson Burnett)
Lord of the Flies
Lord Peter Wimsey
Mary Russell Mysteries
The Merchant of Venice
Merry Melodies/Looney Tunes
Mythbusters
Northanger Abbey
Peter Pan (Disney)
Robert Heinlein World-as-Myth
Rosencrantz and Guildenstern are Dead
RPF-20th-21st Century Politics (Rahm Emanuel)
Sabrina the Teenage Witch
Sarah Connor Chronicles
The Scarlet Pimpernel
Sherlock Holmes
Stargate: Atlantis
Stargate: Atlantis - RPF
Stealing the Elf King's Roses
Supernatural
Terminator
Ten Inch Hero
Tokyo Babylon
Top Gear
Torchwood
Twelfth Night
Ugly Betty
The West Wing
Winnie the Pooh
Young Avengers
Young Wizards
Fics with links, in no particular order
"A Tale of Two Princesses" (Alice in Wonderland/A Little Princess)
"1918" (Sherlock Holmes/Lord Peter Wimsey)
A Unicorn in Central Park (Chrestomanci/How I Met Your Mother)
Conversations in Sussex (Indiana Jones/Laurie R King)
New York Girls (Blue Beetle/Young Avengers)
In Which Worlds Collide and Eeyore Investigates a Terrible Crime (Discworld/Winnie the Pooh)
The Hundred Acre House (House of Leaves/Winnie the Pooh)
Sabrina Spellman and the Witch's Suitcase (Sabrina the Teenage Witch/Harry Potter)
Witternberg Days (Hamlet/Rosencrantz and Guildenstern are Dead)
Deus Ex (Jeeves and Wooster/Classical Mythology/Lord of the Flies/As You Like it)
Libera Me (Constantine/The Dark Knight)
On Ice (Burn Notice/Iron Man)
The Case of the Missing Prince (Disney Princesses - Beauty and the Beast/The Little Mermaid/Peter Pan)
Marvin's Adventures in US Politics (Merry Melodies (Looney Toons)/Torchwood/The West Wing)
Room (FAKE/The Dark is Rising)
Spilling Milk and Falling Stars (Tokyo Babylon/Supernatural)
Witches Can Be Right, Giants Can Be Good (RPF-20th-21st Century Politics (Rahm Emanuel)/Harry Potter.)
Third Time's a Charm (Ten Inch Hero/The West Wing/Supernatural)
Patrick and the Flim-Flam Man (A Dog's Breakfast/Stargate: Atlantis RPF)
The Lion, The Witch and the Chevrolet Equinox (Chronicles of Narnia/Top Gear)
Dressing Up is more Than a Hobby (Beautiful People UK/High School Musical)
Selected Notes on the Mysterious Disappearance of Doctor Kindermann (Die Anstalt/Halloween, Incredible Hulk, Stargate: Atlantis, Terminator)
Beautiful Things (I) (Diane Duane's Stealing the Elf-King's Roses/Young Wizards)
What Keeps the Light (Journeyman/Sarah Connor Chronicles)
The Caged Chameleon (The Scarlet Pimpernel/Brotherhood of the Wolf)
Argosy (Merchant of Venice/Twelfth Night)
Catherine's Adventures in Wonderland (Northanger Abbey/Alice's Adventures in Wonderland)
Spill (Dresden Files TV/BtVS)
The Best Laid Plans of Nanites and Men (Jake 2.0/Ugly Betty)
The Aunt and the Ankh (Jeeves and Wooster/Amelia Peabody)
Next on Mythbusters: Can Luggage Move Itself? (Mythbusters/Discworld)
Facts Not in Evidence (Boston Legal/House MD)
The Eyes of the Observer Make the Mystery (Fringe/Supernatural)
Consider a Box, T, Intersecting a Circle, O (Robert Heinlein World-as-Myth/Doctor Who)