Mirrors appear throughout Sailor Moon, used by Sailor Senshi and foes alike.
Mirrors Used by the Sailor Senshi
Minako Aino/Sailor V/Sailor Venus
Minako Aino's compact had a mirror in it; the mirror only showed the true forms of those who were reflected in it, so it could be used to reveal disguised Dark Agency agents, and always showed Minako's reflection as Sailor V. In Codename: Sailor V, Sailor V used it in many of her attacks, especially Crescent Beam and its powered up versions which utilized light reflected by the compact. This item only appeared in Codename: Sailor V and the Sailor Moon manga.
Michiru Kaiou/Sailor Neptune
Sailor Neptune's talisman was the Deep Aqua Mirror. Like Minako's compact mirror it could reveal hidden truths and was used in the attacks, in this case Submarine Reflection.
Mirrors Used by Enemies of the Sailor Senshi
The Black Moon Clan
The Black Moon Clan made use of mirrors in the R season of the anime. Specifically, Rubeus and the Spectre Sisters used mirrors as portals in their UFO, and Rubeus could communicate with Koan by appearing in her compact mirror.
The Dead Moon
In the SuperS season of the anime, the Dead Moon were searching for Pegasus, who was hidden in the beautiful dream of an individual. To find him the Dead Moon members had to search the Dream Mirrors of humans whom they suspected might be Pegasus' host.
In the Stars season, Queen Nehellenia also used mirrors to trap those who were under her spell. Though this plot element was not present in the manga, there Queen Nehellenia was still trapped within a mirror.