Google Optimization Tools (OR-Tools) is an Open Source software suite for optimization, tuned for tackling the world's toughest problems in vehicle routing, flows, integer and linear programming, and constraint programming.

After modeling your problem in the programming language of your choice, you can use any of a half-dozen solvers to solve it: commercial solvers such as Gurobi or CPLEX, or open-source solvers such as SCIP, GLPK, or Google's GLOP and award-winning CP-SAT.
The suite contains:

● A constraint programming solver;
● A linear programming solver;
● Wrappers around commercial and other Open Source solvers, including mixed-integer solvers;
● Bin packing and knapsack algorithms;
● Algorithms for the Traveling Salesman Problem and Vehicle Routing Problem;
● Graph algorithms (shortest paths, min cost flow, max flow, linear sum assignment).
● Google wrote OR-Tools in C++, with provided wrappers for Python, C#, and Java.

User guides and other downloads can be found here.