JWasm is described as 'Masm v6 compatible assembler. It's released under the Sybase Open Watcom Public License 1.0. JWasm can be used for virtually any purpose, unlike Microsoft's free download of current Masm versions, which are for non-commercial use only'. There are five alternatives to JWasm for Windows, Linux, Haiku and Mac. The best alternative is Pelles C. It's not free, so if you're looking for a free alternative, you could try NASM or Easy code. Other great apps like JWasm are MASM Builder (Free) and Yasm (Free, Open Source).
The Netwide Assembler, NASM, is an 80x86 and x86-64 assembler designed for portability and modularity. It supports a range of object file formats, including Linux and *BSD a.out, ELF, COFF, Mach-O, Microsoft 16-bit OBJ, Win32 and Win64. It will also output plain binary files.
MASM Builder is a free Integrated Development Environment IDE for developing 32-bit Windows programs using the Assembler. MASM Builder contains many useful features which will assist you in creating Windows API based programs.