Google OrTools
Google’s OrTools is an operations research library that is primarily focussed on solving combinatorial programs. Combinatorial problems are widespread in practice: N-Queens; Knapsack problems; Travelling Saleman, etcetera. There are other tools available for other types of programs (e.g.: SOCP, SICP). It is free and open source.
F#
A functional first language which is very expresssive and easy to read/reason. It is built on top of the dotnet framework. Also, have a look at the F# Foundation.