Moody's is infamous for being one of the select few companies that issue bond ratings on government and corporate debt. Along with Fitch Ratings and the Standard & Poor's division of McGraw-Hill ( NYS: MHP ), Moody's issues ratings that institutional investors around the world use to evaluate and price trillions of dollars' worth of securities. The reaction to S&P's downgrade of U.S. government debt is just the latest example of how much investors depend on these ratings.