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
Many users share their work mostly under the LGPL opensource license. Those nodes, not being part of the main executable are called addons and their sources are being hosted on github
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.