I've utilized OR-Tools in conjunction with the CBC (Coin Branch and Cut) solver to solve a mold distribution optimization problem of plastic mold injectors for a client of ours. It was free and it got the job done very well. We used it in C# and it's pretty well documented. We tried LPSolve (55) and it was a pain to use and super slow. Also, commercial options like Gurobi and IBM's CPLEX are super expensive for smaller projects.