Take a Long Line by Derry is a Supernatural/Numb3rs crossover that has me completely hooked after only two (very long and satisfying) chapters. The story starts with Dean in FBI custody, about to be interragated by Don. And it just gets better from there.
One of the many cool aspects of this story is that, in the first two chapters anyway, it's told from the POV of the Numb3rs characters. And if you love seeing the Winchesters through the eyes of "outsiders" (like I do) you'll love this.
The story is Gen, and a WIP (Work In Progress). And, even if you've never seen Numb3rs, I think you'll enjoy this story.