NURBS surface Tesselations

Where can I find any open source code that can tesselate a NURBS surface patch? I have many such pathces stiched together to form the surface. I want to go from this parameteric representation to a simple polygonal rep.

I tried to find some utilities to do this, but they are either not open souce and directly render the tesselations on the screen (e.g. SGI inventor)

Many thanks in advance