Does anyone know a free (as in free beer) C, C++ or even FORTRAN library that supports operations not only NURBS curves and surfaces but also Volumes (i.e. trivariate NURBS)?
I know about open nurbs, but as far as I know it does not support NURBS volumes (feel free to correct me on that, since my experiences with that library are somewhat limited).
IRIT modelling environment supports trivatiate spline volumes.
It is free, but only for non commercial uses. You can contact the developer and ask about the commercial licencing terms.