wxWidgets

Uses custom license that is identical to LGPL except that allows distributions in binary form to use any license people want. Basically you can mod the binary without taking on LGPL responsibilities, patch the binary without taking on GPL responsibilities, or merge the binary to proprietary software and use if anyway you like without taking on GPL responsibilities; just as long as you didn't use the source code to make your mod, patch, or addition. Note that a license of this type allows 3rd parties to sell proprietary bug fix patches and translations (and attach other proprietary based restrictions beyond only cost and unshareability).

The library is in C++ so it's fairly obvious any games using it are also coded in C++ or one of the languages wxWidgets has bindings for.

On Windows, these games usually include wxmsw262u.dll or similar file.