Mere Mortals, by deanine.
I found this AU thanks to mecurtin's recent post tracking WIPs. She obviously has a knack for finding the good stuff on ff.net. The story is long, and projected to be a trilogy, The Lost, Genesis and Identity Crisis. As a whole it is still a WIP, but the first book The Lost is finished. The story has a really interesting premise about Krypton's demise and a lot of original ideas about Krytonite. And it is a true ensemble story with lots of plot and good characterization. I started reading today telling myself that I'd just take a look at it to see whether it was worth keeping track of, and was rapidly hooked by the story. My afternoon and evening went a bit differently than planned, but I was well entertained, so who cares about cleaning and doing dishes?