(written from a Production point of view)
Richard Wagner was a 19th century German composer mentioned in a scene in the script of TNG: "The Inner Light", though only by last name. The scene mentioning Wagner was not in the finished episode. .
Wagner was said to have composed the Ring cycle, which took nine hours to watch in one sitting. Wagner also composed the Bridal Chorus, which was heard in TOS: "Balance of Terror" and VOY: "Course: Oblivion"; he also created the opera Tristan and Isolde, whose title appeared on a banner in VOY: "Future's End, Part II".