LEMON stands for Library for Efficient Modeling and Optimization in Networks. It is a C++ template library aimed at combinatorial optimization tasks which often involve in working with graphs.
LEMON is an open source project. You are free to use it in your commercial or non-commercial applications under very permissive license terms.
If you would like to get to know the library, see LEMON Tutorial.
If you know what you are looking for, then try to find it under the Modules section.
If you are a user of the old (0.x) series of LEMON, please check out the Migration Guide for the backward incompatibilities.