JWasm is a MASM v6 compatible assembler. It's a fork of Open Watcom's WASM 
and released under the Sybase Open Watcom Public License, which allows 
free commercial and non-commercial use. JWasm is written in C, source code 
is open.

JWasm Features:

- JWasm natively supports output formats Intel OMF, MS Coff (32- and 
64-bit), Elf (32-and 64-bit), Bin and DOS MZ.
- precompiled JWasm binaries are available for DOS, Windows and Linux. For 
OS/2 and FreeBSD, makefiles are supplied.
- Instructions up to SSSE3 are supported.
- The JWasm source is portable and has successfully been tested with Open 
Watcom, MS VC, GCC and more.
- As far as programming for Windows is concerned, JWasm can be used with 
both Win32Inc and Masm32.
- C header files can be converted to include files for JWasm with h2incX.

WWW: http://www.japheth.de/JWasm.html