I just stumbled across a fic I think is worth recommending, partially for being a highly unique crossover and partially because so far what I've read is just plain awesome.
Forcibly United by Gaming Ikari takes a "rivals handcuffed together" plot and puts a couple of unique spins on it. For one thing, the rivals in question are Ranma and Ryouga, bound together by magic gauntlets courtesy of Happosai. Second, the key to undoing the gauntlets lies in a far-away city on the Asian mainland... a little place called Roanapur.
Yes ladies and gents, it's a Ranma One Half/Black Lagoon crossover. And it
kicks ass.
I'll be tracking this one from now on; it last updated on 10/3/09 with chapter 15.