Viva the New Year!

In what was sure to be one of the most star-studded concerts of the year, Jay-Z and Coldplay rocked Las Vegas on New Year's Eve, performing at the grand opening of The Cosmopolitan, a $3.9 billion hotel on the famous Strip.

But those two acts weren't the only ones to take the stage: Beyoncé, Kanye West and John Mayer all made surprise appearances during Jay-Z's performance.

Beyoncé joined her husband for "Forever Young" and sang "Halo" with Chris Martin. Mayer played guitar with Jay-Z on his hit song "99 Problems."

"All the problems you were going through have been kicked out the door," Jay-Z told the crowd before a collaborative performance of West's "Homecoming," which featured Jay-Z, West and Martin.

Gwyneth Paltrow was also on hand to watch the show – and ring in the 2011 with her husband and close friends.

The intimate crowd was littered with celebrities to witness the show that Jay-Z called "once in a lifetime." Following their Mexican vacation, Cameron Diaz and Alex Rodriguez were spotted in the audience. Diaz spent some of the evening chatting with Jennifer Lopez and Marc Anthony, not far from Ellen Pompeo and Blake Lively.