The musical episode: 'Once More With Feeling', season six.

Details: In what is probably the definitive musical episode, the town of Sunnydale starts breaking into song when Xander summons a malevolent singing demon.

Standout numbers: 'Under Your Spell', a love song sung to Willow by Tara, who later discovers she's literally under Willow's spell; and (of course) 'They Got the Mustard Out'.