Famous US political prisoner Mumia Abu Jamal was sentenced to death in 1982 in a farce trial with made up evidence that we know today being false. There is a lot of evidence that supports Mumia's innocence, including a confession by another person who admits he was the one to shoot the police officer. But they want Mumia dead he won't shut up, which is the real price demanded by the system for his life.
And the legal system, specially after the Clinton reform, does not allow in practice for re-trials of those sentenced to the death penalty. That is a further obstacle that makes obtaining justice in Mumia's case almost impossible.
However there is nothing that popular action cannot achieve. Mumia's date of execution has been suspended twice already, precisely because popular pressure forced the hand of those who had to sign the execution.
You can sign a petition (already with almost 27,000 signatures) HERE, you can get more information on Mumia's legal and political situation HERE and HERE.
Source of the main info: Amigos de Mumia en Mexico at La Haine[es].