BRENT is a FORTRAN90 library, using double precision arithmetic, which contains Richard Brent's algorithms for finding zeros or minima of a function, without requiring the use of derivatives (or assuming that the function is differentiable.)
ASA047 is a FORTRAN90 library which minimizes a scalar function of several variables using the Nelder-Mead algorithm.
BRENT is also available in a C version and a C++ version and a FORTRAN77 version and a MATLAB version.
COORDINATE_SEARCH is a MATLAB program which minimizes a scalar function of several variables using the coordinate search algorithm.
GSL is the Gnu Scientific Library, a C++ library which includes rootfinding routines.
NELDER_MEAD is a MATLAB program which minimizes a scalar function of several variables using the Nelder-Mead algorithm.
NMS is a FORTRAN90 library which includes versions of Brent's minimizer and zero finder.
PRAXIS is a FORTRAN90 library which minimizes a scalar function of several variables.
SLATEC is a FORTRAN90 library which includes the zero finder FZERO.
TEST_OPT a FORTRAN90 library which defines test problems requiring the minimization of a scalar function of several variables.
TEST_ZERO is a FORTRAN90 library which defines some test functions for which zeroes can be sought.
TOMS178 a FORTRAN77 library which optimizes a scalar functional of multiple variables using the Hooke-Jeeves method.
ZOOMIN is a FORTRAN90 library which includes various zero finder routines.
You can go up one level to the FORTRAN90 source codes.