Only the one for the moment, but it’s a stunningly good crossover, so that’ll do:
This is just about as perfect as comedy crossovers get. Go read, even if you don’t know Blackadder. The Tenth Doctor has to enlist the help of one Edmund Blackadder to stop a marriage. Much sarcasm and hilarity ensues.
(This story is archived on A Teaspoon & An Open Mind.)