Kangaroo is an add-on for Grasshopper/Rhino and Generative Components which embeds physical behaviour directly in the 3D modeling environment and allows you to interact with it 'live' as the simulation is running.
Kangaroo for GC can be downloaded at www.cerver.org in the download section.
Kangaroo is actively being developed by Daniel Piker (http://groups.google.co.uk/group/kangaroophysics) and the GC version maintained by Robert Cervellione.
Downloads:
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Links: