Open market operations are flexible and thus, the most frequently used tool of monetary policy.

http://en.wikipedia.org/wiki/Federal_Reserve_System#cite_note-91

The Federal Reserve education website describes open market operations as follows:

Open market operations involve the buying and selling of U.S. government securities (federal agency and mortgage-backed). The term 'open market' means that the Fed doesn't decide on its own which securities dealers it will do business with on a particular day. Rather, the choice emerges from an 'open market' in which the various securities dealers that the Fed does business with – the primary dealers – compete on the basis of price. Open market operations are flexible and thus, the most frequently used tool of monetary policy.