Will.i.am reflects on his guest appearance during Usher’s Super Bowl Halftime Show after Usher featured in the Black Eyed Peas’ headliner performance in 2011. Then the musician recalls his childhood dreams of being a performer.

These Britney Spears collaborations got that boom boom. For this list, we’ll be looking at the most memorable times the Princess of Pop collaborated with other musicians to create unforgettable tracks. Our countdown includes "Pretty Girls," "Slumber Party...