Sharpe's Dragon by
blackdisa Marvelous crossover between Bernard Cornwells "Sharpe's" and Naomi Novik's "Temeraire" series of novels (and a British TV series in the case of "Sharpe's"). Basically, it's the Napoleonic wars, but with an Aerial Corps of dragons. Best thing I've read in months. Definitely worth reading, even if you're not familiar with the source materiel. Unfortunately, the chapters aren't linked together, so you have to work your way back through the tag pages to read them.
*Edit 1/16/2011: Parts I and II are up at "Archive of Our Own", now:
Sharpe's Dragon Part I and
Sharpe's Dragon Part II.
Midnight Of the Century by
glasslogic Supernatural/Millennium crossover
Pairings: Sam/Jess, Sam/Dean, Sam/Jess/Dean
rating: NC-17
Summary: Sam hasn't spoken to Dean in years. He went to Stanford Law and graduated, now he is studying for the Bar and planning a wedding for the spring. He sees Dean's fetch on Christmas in a churchyard while attending service with his fiancee' Jess and her family. Fetches are death omens for those who will die in the coming year. Suddenly, Sam finds himself on the road, looking for his wayfaring brother and trying to figure out a way for him to avoid his fate.
This story has the best portrayal of Jessica and Sam and Jess's relationship that I've ever come across. And Dean...oh, Dean...