There have been plenty of cases where extreme doubt was cast on the guilt of the executed people. In some the state actually destroyed the evidence after the execution such as Joseph Roger O'Dell where the Prosecutor had claimed, "it would be shouted from the rooftops that ... Virginia executed an innocent man."
In most cases no one, even advocacy groups, are willing to devote time or money on people whom are dead when there are living innocent people awaiting execution who's lives can be saved.
There are a few cases in the U.S. where actual posthumous pardons were given:
Joe Arridy-72 years later
Thomas Griffin-94 years later
Meeks Griffin-94 years later
Lena Baker-60 Years later
I searched pretty thoroughly and these are the only cases I could find of "formal pardons". In each case long after they were dead.
This seems to be the best source for info:
http://www.deathpenaltyinfo.org/Edit:
There are a few older cases aswell:
In 1893, Governor John P. Altgeld pardoned
Albert Parsons,
August Spies,
Adolph Fischer, and
George Engel, who were hanged for their participation in the
May 1886 Haymarket Square riot.
In 2001, Governor Parris Glendening granted a posthumous pardon to
John
Snowden, a Black ice wagon merchant who was hanged in 1919, for the rape and
murder of the wife of a prominent white businessman.
In 1977, Governor Michael Dukakis granted a quasi-posthumous pardon (he
said he would if had the authority, but wasn’t sure he did, but everyone should treat
the statement as a pardon). The recipients of the quasi-pardon were two Italian
immigrant anarchists:
Nicola Sacco and
Bartolomew Vanzetti. They were executed
in 1927 for the robbery and murder of the paymaster and a guard at a shoe factory.
In 1987, Governor Bob Kerrey granted a posthumous pardon to
William
Jackson Marion, on the 100th anniversary of his hanging. Marion was convicted
and executed for the murder of a man who had disappeared but who turned up
alive after Marion was executed.
In 1979, Governor Milton Shapp of pennsylvania granted a posthumous pardon to
Jack
Kehoe, who was executed in 1878 for the murder of a mine foreman.
I think this is all of them 13. This list is just the U.S.
The year of the execution of the last posthumous pardon is 1927.