Giacomo Puccini was a composer of operas.
Dr. Grace Holloway was a fan of Puccini, and listened to his opera Madame Butterfly while she performed surgery on the Seventh Doctor which caused him to regenerate. Later, the amnesiac Eighth Doctor hummed the music to himself, and upon seeing Grace it jogged his memory of the surgery somewhat. Later at Grace's home, seeing her CD of Madame Butterfly further jogged the Doctor's memory of being with Puccini before he died, and of Franco Alfano finishing Puccini's opera Turandot based on his notes. (TV: Doctor Who)
Some time before those events, the Seventh Doctor had found himself trapped in a decompressing airlock as Madame Butterfly played over the speakers, and was determined not to die to the sound of what he angrily called "elevator music." (AUDIO: The Death Collectors)