Open-source library for manipulating 3D triangular surface meshes. Delaunay triangulations, robust geometric predicates, robust boolean set operations on surfaces, surface refinement and coarsening, level-of-detail, k-d trees, volume and curvature, strips generation.
http://gts.sourceforge.net/
http://www.e-insite.net/ednmag/contents/images/31899di.pdf
A Free Surface Reconstructor which turns unorganised point clouds to surfaces. Variety of Output Formats. (Surface Reconstruction Software)
http://www.ams.sunysb.edu/~piyush/reviver
A tool for the algorithmic treatment of polytopes and polyhedra.
http://www.math.tu-berlin.de/polymake/
Free interactive program for modelling liquid surfaces shaped by various forces and constraints, e.g., minimal surfaces (bubbles). Applies gradient descent to minimize energy. Available for most platforms.
http://www.susqu.edu/facstaff/b/brakke/evolver/
http://www.spd.eee.strath.ac.uk/~interact/ztransform/page3.html
Two Java applets for editing polygons and point sets for input to computational geometry software.
http://cgm.cs.mcgill.ca/~godfried/jeoedit/
mathematics and statistics software - S-PLUS - Mathematica
http://www.candiensten.nl/english/