Nixon received a "full, free, and absolute pardon" from Gerald Ford [0], despite not actually having even stood trial, let alone having been convicted.
However, when the president of the United States issues a pardon, even one that is of questionable legality, it's still a powerful statement: Ford placed the power and influence of the Oval Office in between Nixon and any prosecutor.
This petition suggests a similar maneuver with Obama and Snowden.
[0]: http://www.politico.com/news/stories/0909/26847.html