JWasm
Japheth's assembler, JWasm
...As far as programming for MS Windows is concerned, JWasm can be used with both WinInc (32/64-bit) and Masm32 (32-bit). Since v2.01, it will also work with Sven B. Schreiber's ancient WALK32.
C header files can be converted to include files for JWasm with h2incX.
JWasm's source code is released under the Sybase Open Watcom Public License, which allows free commercial and non-commercial use.
There's a bunch of source samples available - they are supplied with the precompiled binary packages.