Justin Bieber, Selena Gomez It's On Again ... Or So it Seems

Justin Bieber and Selena Gomez were at it again, and this time there were unintended beneficiaries ... some random people at a hotel bar who got to hear Justin serenade Selena for free.

The Biebs plopped his butt on a piano stool at the Montage hotel bar in Bev Hills Friday night and sang "My Girl" to his on-again-off-again-on-again-off-again-on-again-off-again GF.

After the mini-concert, they strolled through the empty streets of Beverly Hills.

It's the first time they've been seen together in more than a year.