For anyone in the SGA fandom who hasn't already read it (& thus who must have been either: in a coma, living in a cave or an alternative universe of their own) i'd like to say welcome back, and also to rec the first fic in the Retrograde 'verse...
In the words of the author, Martha Wilson, "The Atlantis expedition has been lost for three years. SG-1 goes to find them, and arrives just a little too late."
So, it's obviously an au, & a crossover, it's also absolutely brilliant. It's one of those fanfics that I can envision as an actual episode (if said episode involved crossing into another universe :P)
Retrograde by Martha Wilson