TNG had Parallels, which while not related to "the Mirror Universe", showed several alternate realities.
I'm blanking on a Voyager equivalent, though. "Deadlock" might qualify.
"Non Sequitur" is more of an alternate reality episode. There are several that dealt with similar concepts of what-might-have-been without actually being alternate universes, like every episode that ever unhappened, "Living Witness," and "Course: Oblivion."