The Fire and the Rose by Abby & Anne. A potions accident leads to an unexpected meeting of minds.
Status: 40 chapters. Complete (plus a long sequel, also complete)
I read this fic several years ago before I started reccing, and only recently decided to reread it. It was a bit of a blast from the past considering what's happened in canon since, but I still enjoyed it quite a lot, even the sex scenes *L* I've always thought other stories should look at this fic before ever attempting a body switching story cause this fic does it perfectly. There's some interesting humour and a pace that since it's either rushing or slowing things down, ends up being just about right. I still think the scene where Snape has to use a tampon is priceless and the ending is great. I wish it didn't have such a cheesy name, but oh well.