As of FEBio 2.9 the licensing has changed. The FEBio source code and Software Development Kit (SDK) are now distributed under the MIT license. The binary executables that can be downloaded from this website are still distributed under a custom license. The main limitation of this license is that the packages downloaded from this website may not be redistributed elsewhere. Also note that the custom license no longer has a restriction for commercial users. The complete license agreement can be viewed here.
PreView executables are freely available for non-commercial use, including academic research and teaching applications. The PreView non-commercial license can be viewed here.