The ACLU's National Prison Project is a national litigation program on behalf of prisoners. Their site has a wealth of information on prisoner's rights in the US.
The Sentencing Project is "a national organization working for a fair and effective criminal justice system by promoting reforms in sentencing law and practice, and alternatives to incarceration.
The Innocence Project works to "to assist prisoners who could be proven innocent through DNA testing."
The Brennan Center's Criminal Justice Program works to "reduce the overuse of incarceration, and to integrate people who have served their sentences into the economic and civic life of the nation so they might lead productive, crime-free lives."
The Sentencing Law and Policy Blog is part of the Law Professors blog network. It "provides Information and Commentary about Guidelines and Capital Sentencing."
The Real Cost of Prisons Weblog is part of a project that "brings together justice activists, artists, justice policy researchers and people directly experiencing the impact of mass incarceration to create popular education materials and other resources which explore the immediate and long-term costs of incarceration on the individual, her/his family, community and the nation."