The AFI has put out a list of the top 100 quotes in movie history. My favorite didn't make the list but Casablanca did have a bunch on there. My favorite quote is from Casablanca when Louie says, "I'm shocked, shocked to find gambling going on in this establishment." Of course this is followed up by a pit boss offering, "here are your winnings, sir."
Here's another one.
AFI List of Top 100 Quotes From U.S. Films - Yahoo! News: "32. 'Round up the usual suspects,' 'Casablanca,' 1942."