GLPK

GLPK (GNU Linear Programming Kit) is intended for solving large-scale linear programming (LP), mixed integer programming (MIP), and other related problems. It is a set of routines written in ANSI C and organized in the form of a callable library. GLPK supports the GNU MathProg language, which is a subset of the AMPL language.
Find GLPK at: http://www.gnu.org/software/glpk/glpk.html