Fall on Your Yesterday's Heart by
Sweet, melancholy story in which Jo travels back in time and winds up entangled in the Winchesters' lives in 1988. Jo and John's relationship is handled nicely, but I especially liked how wee!Sam and wee!Dean (especially Dean) take to her and invite her into the family circle. (There is also, in the narrative present, an allusion to Sam/Dean that was unnecessary and can be skipped if that's not your sort of thing - it's only about a paragraph and it's not graphic.)