Anything by Tom Lehrer.

Especially "Mexico" - the bit in the middle about the guy running over the dog - something to the effect that "it was done with such style and grace that the witnesses awarded him both ears and the tail."

Eric Bogle's "He's Nobody's Moggie Now" is perfect too - I sing it, but the way they do it in Barbershop style is priceless.

Then there's Monty Python - "Every Sperm is Sacred" "Always Look on the Bright Side of Life" and of course the Lumberjack Song (Which I remember teaching to a guide when I was riding horseback in Iceland in 1988 - He hadden't heard about Monty Python, and the song came up when talking to other Americans and we had a devil of a time putting it into context for the gentleman. We also taught him the "Won't You Be My Neighbor" song from Mr. Rogers Neighborhood too. - Try explaining THAT to someone sometime - ya see he's an effeminate man who invites little children into his home and changes clothes in front of them!)