• PuddleDancer Press
This is the webpage for the main publisher of works related to NVC. There are a lot of free resources here (e.g.: articles, newsletters, etc.).
• Empathy Magic - Empathic Listening
H. Holley Humphrey is a long-time teacher of NVC. Her wonderful article on Empathic Listening is available in several languages (i.g.: English, Spanish, French, and German) at the link above.
• The Listening Way
This website is an interesting collection of articles revolving around the work of Carl Rogers, Eugene Gendlin, and Marshall Rosenberg as provided by Jon Russell. Both Eugene Gendlin and Marshall Rosenberg were actually students of Carl Rogers. All of their works are quite complementary with one another. For example, much of Carl Rogers' "Person-Centered Approach" to relationships revolves around treating people compassionately (e.g.: by using "Active Listening" when they are speaking), and Eugene Gendlin's "Focusing" method helps one to get in-tune with their own bodily sensations/feelings. It is easy to see how these can interface with NVC or vice-versa.
• Radical Compassion - Files
Jim and Jori Manske are long-time teachers of NVC. They share some useful resources (e.g.: printable handouts for conducting your own NVC workshops, audio recordings, etc.) at the link above.
• Compassionate Interactions - NVC
This is a fun little site by Jeff Tretsven for learning and applying Compassionate Communication (a.k.a. NVC) through articles and interactive online tools. It also connects it to work by Esther and Jerry Hicks for understanding feelings and needs (i.e.: the "Emotional Scale" and the "Focus Wheel").