There a lot of great ones. Many have already been listed here; Escape from Alcatraz, Sleepers, The Midnight Express, Shawshank, etc.
Not sure if these count as prison movies because the context is military, but:
My contribution is The Last Castle. With Robert Redford, Mark Ruffalo and James Gandolfini. Underrated movie that I feel like flew under the radar, I really liked it.
Also military:
The Great Escape (WW2 POW movie) with Steve McQueen is fucking amazing.
Rescue Dawn (based on a POW true story), with Christian Bale and Steve Zahn is very good too.