MSL is a open source C++ software library for molecular modeling produced in the Senes Lab at the University of Wisconsin-Madison, in collaboration with a team or researchers from a number of Universities and companies.
The main goal is to create a set of tools that enable the computational study of macromolecules with relative ease at all levels, from simple operations (for example, load a PDB and measure a distance or edit a dihedral) to complex applications (protein modeling or design).
The MSL library is not a program (although some applications are distributed) but a tool for scientist to code their own methods.