http://www.truthinjustice.org/p-pmisconduct.htm
I don't vouch for the site but i have heard of some these cases independantly:
"Illinois: In 1994, Chicago cops used a "reverse lineup" (in which a suspect is asked to identify his victims), along with threats and physical abuse, to coerce 17-year-old Lafonso Rollins into confessing to the rape of an elderly woman. He was convicted and sentenced to 75 years prison, but he was freed in 2004 when DNA proved his innocence. He sued. Discovery in his civil suit disclosed that the police crime lab had excluded him based on blood type before Rollins was ever tried. Oops. The great teamwork cost the city $9 million. Cops & Crime Lab, Working Together"
"Ohio: Derrick Jamison has been released from Ohio's Death Row. His 1985 murder conviction was overturned by two federal courts, which ruled he was denied a fair trial by prosecutors who withheld evidence that might have cleared him. 119th Innocent Person Released from Death Row"