Wikipedia
H.A.M.M.E.R. is a fictional espionage and law enforcement agency in the Marvel Comics Universe led by Norman Osborn. The organization is formed in Secret Invasion #8 to replace S.H.I.E.L.D.. The organization plays a large part in Marvel's " Dark Reign" and Siege storylines that ran from 2008 - 2010.
What "H.A.M.M.E.R." stands for, if indeed it stands for anything at all, has not been revealed. In Dark Avengers #1, Osborn told Victoria Hand that it does stand for something, and when she asked what it stands for, he told her to "get on that for him". Also, in the Captain America: Reborn Prelude, when Sin, who is captured by H.A.M.M.E.R, asks what it stands for, the agent present says that it's classified and she does not have a security clearance.