7. Happy Days
Precious few hits enjoy a second life years later as a TV theme. One of them was Bill Haley and the Comets' "Rock Around the Clock," which was the theme of Happy Days for its first two seasons. The rock 'n' roll classic was replaced in 1976 by a similar song penned by Norman Gimbel and Charles Fox. It kept the same '50s rock sound, but the time frame changed. "One o'clock, two o'clock, three o'clock, rock" became "Sunday, Monday, Happy Days." The Cunningham family, and their friend Fonzie, didn't miss a beat.