The 15 Best Hairstyles of the Past Decade

We're not sure exactly when Rihanna's "undercut" hairstyle became insanely popular this year (who wakes up thinking, Hey, I'd really like to shave some of my head today?), but we'll admit that there is something sexy about the edgy, unexpected haircut. Celebs like Keri Hilson and Cassie and young beauties like Willow Smith embraced the half-shaved hair trend and give off an irresistible rocker-cool vibe.