"Vesti la Giubba" is a famous aria from the opera Pagliacci— and one of my favorites. It's been used in so many things, I bet everyone's heard it at one time or another.
The above clip is sung by Franco Corelli. He is amazing.
For those who aren't familiar with the opera, this scene is when Canio discovers his wife's infidelity, but must nevertheless prepare for his performance as Pagliaccio the clown because "the show must go on."
Smiling on the outside, crying on the inside.
If you want to jump right to the aria, it starts at 2:43.