The revelation that Fox News reporter James Rosen was tracked by the government in their effort to prosecute Stephen Jin-Woo Kim for passing "national defense information" (the ill-defined term used in the Espionage Act) comes on the heel of news that the Department of Justice seized a number of phone records from the AP in an effort to root out another leaker. The president insists he believes in an "unfettered" free press, but that nevertheless the government must balance that with the interests of "national security." So what information was Kim accused of passing? That North Korea might launch more nuclear tests in response to more sanctions, in 2009. The North Korean regime has conducted several nuclear tests in the intervening years.
In the AP case, reports suggest the Department of Justice is trying to ferret out the leaker behind a story about an alleged foiled terrorist plot in May 2012. News of the plot ran contradictory to the campaign narrative that Al-Qaeda posed no threat. The White House asked the AP to withhold the story until the government could announce the foiled plot itself; the AP waited until concerns about the ongoing intelligence operation were allayed. But it appears the source of the information that the plot was an inside job by a double agent could've been John Brennan, then the president's chief counterterrorism advisor, who told a conference call of talking heads the plot was never a threat to the U.S. because there was "insider control" of it.
As for the national security merits of other leakers prosecuted under the Obama administration, one of the first, Thomas Drake, was indicted in April 2010 for "mishandling" documents related to the leaking of information about the NSA's data collection efforts. The original charges were all dropped a year later. Also in 2010, a former FBI translator, Shamai Leibowitz, was sentenced to 20 months in prison for passing unidentified documents (even the presiding judge didn't know what was in them) to an unidentified blogger. Leibowitz said he believed the documents showed the government broke the law. In January 2011, Jeffrey Sterling was charged with passing information to a reporter about a Clinton era effort to sell bad nuclear weapons designs to Iran.
In January 2012, John Kiriakou, the first government official to acknowledge CIA waterboarding, became the sixth person during Obama's time in office charged under the Espionage Act for passing the names of CIA interrogators to journalists. Kiriakou eventually pled guilty and was sentenced to 30 months in jail.
The most well-known of the Obama administration's anti-leaks cases is the one against Bradley Manning, who President Obama announced had "broke the law" before any trial had begun Manning is accused of facilitating the largest leak in U.S. history, a cache of State Department cables and material related to the Iraq and Afghanistan wars published by Wikileaks. The judge in the Manning case has told prosecutors they will have to prove not just that Manning leaked the information, but that he knew he was aiding the enemy. In 2010, the Pentagon said none of the documents published by Wikileaks jeopardized U.S. intelligence or military operations, and a 2011 review by AP found that sources identified in leaked documents were not threatened. Manning's trial is set to begin next month.
The New York Times last year provided a run down of these six cases and the three like it that occurred before Obama's term in office, and here's a 1986 Time magazine article on Reagan's campaign against leakers (which yielded one of the three pre-Obama cases, that of Samuel Morison, who was pardoned by Bill Clinton in 2001).