People and Things, by karaokegal, is a Torchwood/House crossover that can perhaps be best summed up as follows:
Rough Heals, by voleuse, is recommended everywhere, and with good reason. This is Dawn in the FF-'verse -- a Dawn who never died, never grew old -- and it's every bit the tragedy it ought to be. Beautiful and melancholy. (gen; BtVS/Firefly)
Game Theory, by halcyon_shift, is a five-part Highlander/Numb3rs crossover. I've been waiting to read one of these for a while, and this one gets bonus points for mentioning Matthew McCormick. Well-plotted, well-written, and nicely characterized. (gen; HL/Numb3rs; link is to the first part.)
The White Lights and the Blue, by giddygeek is a Hot Fuzz fic, with a surprise crossover at the end. The relationship between Nick and Danny is nicely done here, and the mystery is light-hearted and fun. (slash; Nick/Danny)
When Sam Met Sam (and Dean), by elayna88, is a Supernatural/Reaper crossover that is fairly well summed up by the title. If you haven't watched Reaper yet, give it a try. It's pretty entertaining - as is this story. (gen)
Life in Little Canada, by laytoncolt, is absolutely fucking hilarious. It's a Stephanie Plum/SGA fusion-fic (yes, you read that correctly), and it's got the same special twist of lunacy as do the books -- but with an SGA flavoured twist. Read it, if only to see Kavanaugh as Vinnie, and Rodney in the Stephanie Plum role. (slash; John/Rodney)
My name is Rodney McKay, and I grew up in Little Canada. I know what you're thinking, you're thinking there's no such place. I promise you, it exists. I would not have made this place up. It's in Minnesota, just a little town by Minneapolis that you'd miss if you blinked.
The residents of Little Canada have so valiantly hung onto their heritage that they have become far more concerned with being Canadian than actual Canadians. The Canadian flag hung proudly at least at one house out of three, and little girls sold maple syrup on street corners instead of lemonade. The façade is all so complete that I had actually believed I was living in Canada until the age of six, when I learned that those crazy and mysterious Americans my father was always ranting about actually included myself.
The homes are well kempt and everyone always smiles and says thank you and offers to feed your cats when you're away. They love the idea of the nice guy Canadian stereotype, but the truth is they probably only want to feed your cats so they can go through your things when you're not around. They catch tourists off their guard, smiling bright and wide as they overcharge the unsuspecting morons under the guise of keeping their prices at the true Canada rate.