masm compatible assembler.
Runs under Windows, Linux, DOS, OS/2 ( and probably other OSes as well ).
Hints:
For Windows, use file Msvc.mak/Msvc64.mak if Visual C++ is to be used to create a 32-/64-bit version of JWasm. OWWin32.mak will create a 32-bit binary using Open Watcom.
For Linux, use GccUnix.mak to produce JWasm with gcc or CLUnix.mak to use CLang instead.
For DOS, Open Watcom may be the best choice. It even allows to create a 16-bit (limited) version of JWasm that runs on a 8088 cpu. Old versions of Visual C++ are also possible, although you probably need the HX development files then.
For OS/2, OWOS2.mak (Open Watcom) is supplied.
There is a bunch of other makefiles in the main directory, intended for other compilers. Some of them might be a bit outdated.