A free cross-platform library of fast C++ routines for the plotting of up to 3-ranged data. It can export to bitmap and vector EPS/SVG files. There are window interfaces (GLUT/FLTK/Qt) and console tools. MathGL can be used from C/Fortran/Python/Octav/Lua
Netkit is a network emulation environment that allows the user to setup complex scenarios on a single PC with little effort and at no cost. In itself, Netkit aims at integrating different other open source products to provide an easy-to-use interface.