For some reason I've never been able to get into Baroness, although I've heard really good things. And they always seemed to get mentioned along side Mastodon.
Exanimate, if you like Nachtmystium you might like Lord Mantis (they share 2 members). However, LM is pretty sick and not for normal folks :-* Nachtmystium is a different band but I really dig their track Every Last Drop.
And you can't forget the new YOB (featuring Scott Kelly from Neurosis):
I also really dig this new doom band Pallbearer: