Masm32

Masm32 is a software packet which includes tools to build Win32 applications with Masm. It contains:

Masm v6.14, Link, RC - tools from Microsoft (originally contained in the Windows 98 DDK).
WINDOWS.INC - include file in Masm syntax declaring Win32 equates, typedefs and structures
MACROS.ASM - a macro "library" ( slightly inspired by BASIC ).
KERNEL32.INC/USER32.INC/... - contain prototypes for Masm's INVOKE directive
KERNEL32.LIB/USER32.LIB/... - import libraries in MS-COFF format
MASM32.LIB - a library of simple functions. Source (Masm syntax) is included.
some tools and source code samples

The Masm32 licence is limited to create binaries for MS Windows (see details here), which sounds strange and might limit its usefulness. The license also tells that Masm32 cannot be used to create "open source" software ( which is pure nonsense of course ).

There's also a forum for Masm32. Be aware that it isn't a generic "assembly language forum", it's rather about programming Win32 applications in (Masm-style) assembly.