http://en.wikipedia.org/wiki/List_of_United_States_presidential_assassination_attempts_and_plotsAndrew Jackson
January 30, 1835: At the Capitol Building, a house painter named Richard Lawrence aimed two flintlock pistols at the President, but both misfired, one of them while Lawrence stood within 13 feet (4 m) of Jackson, and the other at point-blank range. Lawrence was apprehended after Jackson beat him down with a cane. Lawrence was found not guilty by reason of insanity and confined to a mental institution until his death in 1861. The odds of two consecutive misfires were estimated at 1 in 125,000.
Abraham Lincoln
February 23, 1861: The Baltimore Plot was an alleged conspiracy to assassinate President-elect Abraham Lincoln en route to his inauguration. Allan Pinkerton, eponymous founder of the Pinkerton National Detective Agency, played a key role by managing Lincoln's security throughout the journey. Though scholars debate whether or not the threat was real, clearly Lincoln and his advisors believed that there was a threat and took actions to ensure his safe passage through Baltimore.
Theodore Roosevelt
October 13, 1912: Three and a half years after he left office, Roosevelt was running for President as a member of the Bull Moose Party. In Milwaukee, Wisconsin, John F. Schrank, a saloon-keeper from New York, shot Roosevelt once with a .38 caliber revolver. A 50-page speech folded over twice in Roosevelt's breast pocket and a metal glasses case slowed the bullet. Amidst the commotion, Roosevelt yelled out, "Quiet! I've been shot." Roosevelt insisted on giving his speech with the bullet still lodged inside him. He later went to the hospital, but the bullet was never removed. Roosevelt, remembering that William McKinley died after operations to remove his bullet, chose to have his remain. The bullet remained in his body until his death. Schrank said that McKinley's ghost had told him to avenge his assassination. Schrank was found legally insane and was institutionalized until his death in 1943.
Franklin D. Roosevelt
On Thursday, February 15, 1933 in Miami, Florida, Giuseppe Zangara fired five shots at Roosevelt. The assassination attempt occurred one month before Roosevelt was sworn in for his first term in office. Four people were wounded and Chicago Mayor Anton Cermak was killed. Zangara was found guilty of murder and was executed March 20, 1933. Some researchers believe Cermak, not Roosevelt, was the intended target that day, as the mayor was a staunch foe of Al Capone's Chicago mob organization.
Harry S Truman
In 1950, two Puerto Rican pro-independence activists, Oscar Collazo and Griselio Torresola, attempted to kill Truman. A violent gun battle ensued between the assassins and the Secret Service, resulting in the death of White House Policeman Leslie Coffelt. Coffelt was able to kill Torresola before blacking out and soon dying. Collazo survived with serious injuries. Truman was not harmed.
John F. Kennedy
December 11, 1960: While vacationing in Palm Beach, Florida, President-elect John F. Kennedy's life was threatened by Richard Paul Pavlick, a 73-year-old former postal worker. Pavlick's plan was to serve as a suicide bomber by crashing his dynamite-laden 1950 Buick into Kennedy's vehicle, but the plan was disrupted when Pavlick saw Kennedy's wife and daughter bidding him goodbye. That attack of conscience foiled the opportunity, with Pavlick's arrest by the Secret Service coming three days later after he was stopped for a driving violation, with the dynamite still in his car. Pavlick spent the next six years in both federal prison and mental institutions before being released in December 1966.
Richard Nixon
February 22, 1974: Samuel Byck apparently planned to kill Nixon by crashing a commercial airliner into the White House. Once he had hijacked the plane on the ground, he was informed that it could not take off with the wheel blocks still in place. He shot the pilot and copilot, then was shot by an officer through the plane's door window before killing himself. The events surrounding this assassination attempt were portrayed in the film The Assassination of Richard Nixon.
Gerald Ford
September 5, 1975: In Sacramento, California, Lynette "Squeaky" Fromme, a follower of Charles Manson, drew a Colt .45 caliber pistol on Ford when he reached to shake her hand in a crowd. There were four cartridges in the pistol's magazine but the firing chamber was empty. She was soon restrained by a Secret Service agent. Fromme was sentenced to life in prison, but was released from custody on 14 August 2009.
September 22, 1975: In San Francisco, California, Sara Jane Moore fired a revolver at Ford from 40 feet (12 m) away. A bystander, Oliver Sipple, grabbed Moore's arm and the shot missed Ford. Moore was sentenced to life in prison. She was later paroled on Monday, December 31, 2007, from a federal prison after serving more than 30 years.
Jimmy Carter
May 5, 1979: Two minutes before Carter was about to speak at the civic center mall in Los Angeles, Raymond Lee Harvey was arrested carrying a pistol. He later told authorities that he and another man, Osvaldo Ortiz, were hired to create a diversion so that Mexican hit men armed with sniper rifles could kill Carter. Charges against him were dismissed for lack of evidence.
Ronald Reagan
On March 30, 1981, following a speaking engagement at the Hilton Washington Hotel in Washington, D.C., Reagan and three others were shot and wounded by John Hinckley, Jr., as Reagan was returning to his limousine. The others that were shot were White House Press Secretary James Brady, Secret Service agent Tim McCarthy, and D.C. police officer Thomas Delahanty, all of whom survived, though Brady was permanently disabled. Reagan survived and recovered after emergency surgery at nearby George Washington University Hospital.
George H.W. Bush
April 13, 1993: Sixteen men, in the alleged employment of Saddam Hussein's Iraq, smuggled a car bomb into Kuwait with the intent of killing Bush as he spoke at Kuwait University. The supposed plot was foiled when Kuwaiti officials found the bomb and arrested the suspected assassins. Bush had left office in January 1993. On June 26, 1993, the U.S. launched a missile attack targeting Baghdad intelligence headquarters in retaliation for the attempted attack against Bush. The Iraqi Intelligence Service, particularly Directorate 14, is thought to be behind the plot, though other sources say evidence is thin.
Bill Clinton
September 12, 1994: Frank Eugene Corder flew a single-engine Cessna into the White House lawn, apparently trying to hit the White House. The President and First Family were not home at the time. Corder was the only casualty.
October 29, 1994: Francisco Martin Duran fired at least 29 shots with a semi-automatic rifle at the White House from a fence overlooking the north lawn, thinking that Clinton was among the men in dark suits standing there (Clinton was in the White House Residence watching a football game). Three tourists, Harry Rakowsky, Ken Davis, and Robert Haines, tackled Duran before he could injure anyone. Duran was found to have a suicide note in his pocket and was sentenced to 40 years in prison.
November 1994: In anticipation of Clinton's visit to Manila in the Philippines, al Qaeda operative Ramzi Yousef planned to blow up his motorcade. The plan was aborted due to heavy security
George W. Bush
February 7, 2001: While President George W. Bush was occupied in the White House Residence, Washington, DC, Robert Pickett, standing outside the perimeter fence, discharged a number of shots from a weapon in the direction of the White House. The U.S. Park Police claimed, according to CNN correspondent Eileen O'Connor, that the type of handgun that was confiscated was of a sophisticated type and had the shooter not been shooting from an obstructed angle view, the bullets would have reached the White House. However, numerous trees and bushes separated the sidewalk, where Pickett was, from the White House. Following a stand-off of about ten minutes, the incident ended when a Secret Service officer shot Pickett, resulting in an injury which required immediate hospital surgery. Pickett was found to have emotional problems and employment grievances. Although Pickett had written letters to the President about these grievances there was lack of conclusive evidence that President Bush was a personal target. A court in July 2001 sentenced Pickett to three years imprisonment in connection with the incident.
On the morning of September 11, 2001, Bush was at the Colony Beach and Tennis Resort on Longboat Key, Florida. He woke up around 6:00 AM and prepared for his morning jog. A van occupied by men of Middle Eastern descent arrived at the Colony Beach Resort and claimed they had a "poolside" interview with the President. They did not have an appointment and were turned away. It is possible this was an assassination attempt modeled on the one used on anti-Taliban fighter and Northern Alliance military leader Ahmed Massoud two days earlier. The previous April, Massoud addressed the European Parliament and warned of the possibility of al-Qaeda attacking in the West. Longboat Key Fire Marshal Carroll Mooneyhan was reported to have overheard the conversation between the men and the Secret Service, but he later denied the report. The newspaper that reported this, the Longboat Observer, stands by its story. Both Mooneyhan and the Observer reporter were questioned by the Secret Service, but the agency has not commented further. Witnesses have recalled seeing 9/11 hijacker ringleader Mohamed Atta in the Longboat Key Holiday Inn a short distance from where Bush was staying as recently as September 7, the day Bush’s Sarasota appearance was publicly announced.
May 10, 2005: While President George W. Bush was giving a speech in the Freedom Square in Tbilisi, Georgia, Vladimir Arutyunian threw a live Soviet-made RGD-5 hand grenade towards the podium where he was standing and where Georgian President Mikhail Saakashvili and their two wives and officials were seated. The grenade was live and had its pin pulled, but did not explode because a red tartan handkerchief wrapped tightly around the grenade kept the firing pin from deploying quickly enough. Arutyunian was arrested in July 2005, and killed an Interior Ministry agent while resisting arrest. He was convicted in January 2006, and was given a life sentence.
A you can see, the attempts were rare over the years (relatively) till Kennedy was shot. Since then, there have been attempts made on the lives of every sitting President except for Johnson, though Reagan has been the only one injured. The most recent attempt on American soil, by an American citizen, was against Bush II in 2001, a mere 9 years ago.
There's more at the Wikipedia page, including the "successful" plots, and a bit about deaths that were rumored to be assassinations.
http://en.wikipedia.org/wiki/List_of_United_States_presidential_assassination_attempts_and_plots