World's most popular song: "Happy Birthday to you" by Hill Sisters

World's most popular song: "Happy Birthday to you" by Hill Sisters
http://en.wikipedia.org/wiki/Happy_Birthday_to_You

On Piano:
http://in.youtube.com/watch?v=hrvA1AEL7zA

Others:
Beatles:http://in.youtube.com/watch?v=glNjsOHiBYs&feature=related
Elvis:http://in.youtube.com/watch?v=5FO7XaDs_8Y&NR=1
http://in.youtube.com/watch?v=WmiFWG1Ox1Y&feature=related
Animated:
http://in.youtube.com/watch?v=UeypOvsY91Q&feature=related
Songs on same theme from other languages and countries:
http://en.wikipedia.org/wiki/List_of_birthday_songs