Shout-out to Vee's lists about other popular pieces!
The "Dies Irae" is a piece of music originating in the Roman Rite liturgy for the Requiem Mass. You've probably heard it before in classical music or as part of a film soundtrack. Some examples: Gregorian Chant, Berlioz, Shining.
Suggestions and corrections welcome! I'll try to provide links in the notes.
Other lists: Mozart's version, Verdi's version