ROOT is a data analysis framework.
SageMath is a free open-source mathematics software system licensed under the GPL.
ScaLAPACK is a library of high-performance linear algebra routines for parallel distributed memory machines.
SCOTCH is a software package and libraries for sequential and parallel graph partitioning, static mapping, and sparse matrix block ordering, and sequential mesh and hypergraph partitioning.
SPECFUN is a FORTRAN90 library which evaluates certain special functions, by William Cody and Laura Stoltz.
SPRNG provides the user the various SPRNG random number generators each in its own library.
SuiteSparse is a suite of sparse matrix algorithms.
SuperLU contains a set of subroutines to solve a sparse linear system A*X=B.