"Happy Birthday to you", the default birthday song across the planet, not to forget the most well-known, is a copyright work! That's right: you use it commercially without prior permissions and you get the stick.
Wikipedia, Kuro5hin and Snopes provide more information on this little known fact. Its genesis dates back to 1893, when two sisters in Kentucky collaborated to create a small melody for wishing their kindergarten students a goodmorning. Through adaptations, the lyrics changed to ones that we know of today, and through a lucky court ruling, the copyrights went to the sisters. Warner Chappell Records currently holds the rights to the song, so beware of the big label scare next time you use it. The rights expire in 2030.
Have fun watching Ms. Marilyn Monroe performing this song on JFK's birthday in '62.