Five eco lightbulb jokes here (P.S. and Robert Butler is looking for more. My contribution would be a variation on an old one: "How many climate sceptics does it take to change a lightbulb? None, because climate sceptics aren't afraid of the dark", only someone beat me to it).
Quite a good green joke is one of the cartoons on the letters page of the 10 May New Scientist. A woman is walking away snootily from a man who is holding out some flowers and saying "I bought you these solar-powered devices for carbon dioxide extraction".
Making you laugh, then making you drink are Last words.
But most profound is Tom Waits.