Max.NET is a .NET CLI wrapper of (almost*) the whole 3d Studio Max SDK. It gives anybody the ability to use managed environment and a .NET language (such as C#, Visual Basic, J#, and many more) to extend 3d Studio Max by writing plugins and tools that exist within its environment.
Because Max.NET provides a 1:1 wrapped translation of the C++ SDK it is possible to do almost anything that can be done through C++. The managed environment, however, provides a number of benefits over the unmanaged C++. Some of these benefits are:
The speed of plugins and tools using Max.NET is comparable to that of C++. This means that these tools will run (much) faster than any MaxScript counterpart.
Max.NET is free for non-commercial use given that you also provide source code for your plugins if you are distributing them (as per free GPL license). A commercial license must be obtained for any other use. This license grants unlimited commercial development as well as support time and full source-code for Max.NET.