CasADi is a symbolic framework for numeric optimization implementing automatic differentiation in forward and reverse modes on sparse matrix-valued computational graphs. It supports self-contained C-code generation and interfaces state-of-the-art codes such as SUNDIALS, IPOPT etc. It can be used from C++, Python, Matlab/Octave, Julia or Javascript
Kratos Multiphysics (A.K.A Kratos) is a framework for building parallel multi-disciplinary simulation software. Modularity, extensibility and HPC are the main objectives. Kratos has BSD license and is written in C++ with extensive Python interface.
:spider_web: A Python interface to CGAL's meshing tools
2D/3D simplicial mesh generator interface for Python (Triangle, TetGen, gmsh)
Plotly for C++. A C++ interface to the Plotly.js figure spec, for creating interactive data visualizations.