Truthfully, I don't read many crossover fics, but here are a few that caught my eye...
- I Think Your Gun is Bigger than Mine Hunters are good at recognizing other predators. This is Dean/Alexis Drazen (Misha) from 24.
- Yeti and Angels Howard has been taken by Yeti, and Naboo knows just who to call... A Supernatural/Mighty Boosh crossover.
- Breathing Through the Radio Supernatural/Welcome to Night Vale crossover. Falling means loneliness, isolation and having a terrible and crippling emptiness inside. And when you feel like this, so hopeless and desperate, even a disemboweled and mildy creepy voice that narrates the life of an ever creepier desert town can be a comfort; aka, after he falls, Castiel starts hearing the Night Vale community radio program playing in his head, helping him, keeping him company and making him realize the depth of his feelings for Dean; and lets be real, this is not even the weirdest thing that ever happened to him.
- SuperNewGirl Sam and Dean work a case in Los Angeles and run into the New Girl gang. Dean has a sudden (and short lived) belief in love at first sight.