The Innocence Project

"The Innocence Project was founded in 1992 by Barry C. Scheck and Peter J. Neufeld at the Benjamin N. Cardozo School of Law at Yeshiva University to assist prisoners who could be proven innocent through DNA testing. To date, 268 people in the United States have been exonerated by DNA testing, including 17 who served time on death row. These people served an average of 13 years in prison before exoneration and release."

http://www.innocenceproject.org/

 "Unvalidated or improper forensic science is a leading cause of wrongful convictions. In more than 50% of the DNA exonerations nationwide, unvalidated or improper forensic science contributed to the underlying wrongful conviction."

According to the University of Michigan Law School which has an Innocence Clinic (here) these are the leading causes of wrongful conviction: 

Eyewitness Misidentification Eyewitness misidentification is the single greatest cause of wrongful convictions nationwide. Research shows that the human mind is not like a tape recorder; we neither record events exactly as we see them, nor recall them like a tape that has been rewound. Instead, witness memory is like any other evidence at a crime scene; it must be preserved carefully and retrieved methodically, or it can be contaminated. 

Junk Science Many forensic testing methods have been applied with little or no scientific validation and with inadequate assessments of their significance or reliability. As a result, forensic analysts sometimes testify in cases without a proper scientific basis for their findings. And in some cases, forensic analysts have engaged in misconduct.

False Confessions In many cases, innocent defendants make incriminating statements, deliver outright confessions, or plead guilty. Regardless of the age, capacity, or state of the confessor, what they often have in common is a decision—at some point during the interrogation process—that confessing will be more beneficial to them than continuing to maintain their innocence.
Government Misconduct In some cases, government officials take steps to ensure that a defendant is convicted despite weak evidence or even clear proof of innocence.

Snitches Often, statements from people with incentives to testify—particularly incentives that are not disclosed to the jury—are the central evidence in convicting an innocent person. People have been wrongfully convicted in cases in which snitches are paid to testify or receive favors in return for their testimony.

Bad Lawyering The failure of overworked lawyers to investigate, call witnesses, or prepare for trial has led to the conviction of innocent people.

According to Samuel R. Gross, professor at the University of Michigan Law School, it is statistically likely that as many as 80,000 innocent people have been convicted since 1980.