Selena Gomez, Taylor Swift
Credit: Larry Busacca/Getty Images

After a day of shopping in New York City, AMA Artist of the Year Taylor Swift welcomed Selena Gomez on stage during the Madison Square Garden stop on her Speak Now world tour, where the duo sang Gomez's hit "Who Says” together. "Performing at Madison Square Garden with my best friend was a highlight," Gomez tweeted after taking the stage. Click through to see Elle Fanning, Salma Hayek and more stars at last night hottest parties!