An international list:

Bye Bye Blackbird
The White Cliffs of Dover
Danny Boy
Tipperary
Juantanamera
Will you go, lassie, go
Molly Malone
Tulips from Amsterdam
Lily Marlene
Je ne regrette rien
Just one cornetto (help, what's the real title?)(O sole Mio)
Girl from Ipanema
Sweet Chariot
Summertime
Edelweiss
Yellow Submarine
Waltzing Matilda

These are not personal favourites, but everybody in Europe, and presumably North America too, knows at least the melodies to these songs, and they're not too difficult to sing (unlike many listed above, eg Whiter Shade of Pale).