All Whedonverse crossovers (BtVS/Angel, Firefly, Dollhouse, Dr. Horrible), all the time.
About Little Black Dress (littleblackdress)
So, the Jossverse and its characters cross really well into other texts. The mods wanted to celebrate that fact (and, okay, make it easier to feed their addiction). And so the Little Black Dress Collection was born.
All stories must involve a Joss Whedon source text. Other than that, go nuts.