4. Justin Timberlake and Garth Brooks
Bridgestone Arena, December 19th
What goes around, comes around. At a sold-out Garth Brooks show on December 4th, fans were entertained not only by the action onstage, but also the antics of a particularly enthusiastic fan in the front row. That person was Justin Timberlake, who'd decided to enjoy a short break in his own touring schedule by flying to St. Louis and hitting up Brooks' concert. Two weeks later, Brooks returned the favor by walking onstage at Timberlake's arena show in Nashville. Backed by a massive backup band and more than 15,000 screaming fans, the two sang "Friends in Low Places" together, turning a lonely man's drunken lament into a show-stealing duet.