Netflix has put When They See Us, Ava DuVernay's 4-episode mini-series about the Central Park Five, in front of their paywall for free viewing. Here's the trailer:
The 2013 Ken Burns documentary The Central Park Five is available to watch on the PBS site and also on Amazon.
As previously noted, DuVernay's 13th and Selma are also both available to watch online for free.