Previous Post There was the "Mirror, Mirror" ep from TOS, then one in Enterprise where evil-Archer somehow got TOS Enterprise's ship (or something like that, been awhile since I saw it), and now IDW Publishing is currently doing one (LS) for TNG called, "Star Trek: The Next Generation: Mirror Broken" (waiting for the TPB). Are there any others besides these?

"Mirror, Mirror" was the only TOS episode to feature it.

TNG managed to avoid encountering the Mirror Universe on the show, but there was at least one book written about it, "Dark Mirror."

DS9 made the most hay with the Mirror Universe. Season 2's "Crossover" saw Kira and Bashir tumble into the Mirror Universe where the station was run by an evil Kira. Season 3's "Through the Looking Glass" takes Sisko into the MU to impersonate his dead doppleganger. Season 4's "Shattered Mirror" has Sisko go to the MU to retrieve a kidnapped Jake and help the Human rebels outfit their own Defiant. Season 6's "Resurrection" involves a plot by evil Kira on board DS9. Season 7's "The Emperor's New Cloak" has Grand Nagus Zek in the MU being rescued by Quark and Rom who run into the MU's evil Worf.

Enterprise's "In a Mirror, Darkly" was a two-parter which also involves time travel, the Tholians, and retroactively inserting themselves into a TOS episode "The Tholian Web."

I'm sure there are a million more novels and comics which employed the Mirror Universe. Like that trilogy of novels written by Shatner about Emperor Tiberius (Mirror Kirk).