Three to start with, repeated from the post in my journal.
Harry Potter (Ron/Draco-swimming lessons): Postwar; the Malfoys have been forced into a certain amount of public service as reparations. They hold summer swimming lessons for children at their heated pool. By a twist of legislature, they manage to insist that only purebloods can attend. (Maybe there's a pureblood-only charm on the gate that they can't get rid of.) Ron brings Harry's son for lessons. Ron & Draco barely manage to be civil, until James almost drowns and Draco rescues him. Any slasher knows what happens next.
Firefly (River/Jayne-secret meetings): In the middle of the night, River crawls into Jayne's bunk, fucks him like crazy, then goes away. The next day, she doesn't mention it, and Jayne's not sure if she even remembers it. Next night, it happens again. Next day, again, she doesn't acknowledge it. This continues. Jayne can't figure out if she's really hot for him and not willing to admit it during the day, or if she's so out of her mind she really doesn't remember. He's also not sure if he wants it to continue, cos on the one hand, SEX, on the other, CRAZY GIRL (with insanely overprotective brother who'll go psycho on him if he finds out; plus, she can kill him with her brain).
House/X-Files crossover (gen-werewolves): Mulder winds up tracking a werewolf rumour in New Jersey; the guy in question is submitted to PPTH with a pack of symptoms that say "werewolf" to Mulder and "lupus/sarcoidosis" to House's crew, except for the silver allergy and irregular hair growths. House manages to fix the silver problems and the guy disappears just before the full moon. (Optional slash between Mulder and Wilson. Bonus points for making Wilson one of the doctors consulted on Scully's cancer, so they've met before.)