In no particular order:
Any number of songs from Blazing Saddles (I'm Tired, I Get No Kick From Champagne, The French Mistake,
A Man of Constant Sorrow
, performed twice in O, Brother, Where Art Thou?
Why Don't You Do Right?
performed by Jessica Rabbit in Who Framed Roger Rabbit.
Knights of the Round Table
from Monty Python and the Holy Grail.
And, as previously mentioned, I loved the dwarves' song in The Hobbit.