Pastoral Care Units at local prisons are always in need of volunteers who can provide a quick visit or perhaps teach a skill to those who are incarcerated. Learning job skills is paramount for these men and women so they will have marketable skills when they get out of prison.
You might also consider writing to a prisoner — sharing prayers with him/her and simply a general concern for their well-being. WriteAPrisoner is just one of many programs that link people with a prisoner pen pal.
Work for the abolition of the death penalty. Use resources from Catholic Mobilizing Network to help you.
Make prisons a priority for your parish community. We can all too easily forget about those who have been placed behind bars. Start with regular prayer for prisoners and raise awareness about the prison facilities in your local community.