vvvv itself is not open source, but it offers different open interfaces for people to write their own nodes:
Plugins: most general interface for writing multipurpose nodes
Freeframes: video effects/analysis nodes
Effects: vertex- and pixelshader
Modules: reusable vvvv-patches
VST: audio effects/analysis nodes
All contributed addons are available via a separately downloadable addonpack in order to allow independent release cycles of the main vvvv application and user submitted content.