JCarlos, quotes by Joseph Stiglitz sounds like a good idea to me. I think he'd make a worthy addition to this site.
"If stability and efficiency required that there existed markets that extended infinitely far into the future - and these markets clearly did not exist - what assurance do we have of the stability and efficiency of the capitalist system?"--Joseph Stiglitz
Spellbinder, you'll be glad to know that one of your Marx quotes is already used on this site.
"I refuse to join any club that will have me as a member."
Actually, this quote was said by Groucho and not Harpo. Some of the most famous Harpo Marx quotes include:
"[loud, shrill whistling]"
"[blows antique auto horn]"