I just refound an awesome fic that I've been looking for forever! It's a House, MD/Joan of Arcadia crossover. I first read it before I'd ever seen House, so I didn't have a full appreciation of it then... but now!
For anyone (probably most people) who never saw Joan of Arcadia, it was a show that - surprise- had a girl that had conversations with God... and it teetered a bit on the 'is she crazy or really seeing him' line. Anyway, God was always appearing as a different person, so anyone she met could be God...
Hallowed Be Thy Greg by SuzvoyFunny fic - House is his normal self and Joan thinks he's God...