JWasm

JWasm is a free MASM-compatible assembler with these features:

native support for output formats Intel OMF, MS Coff (32/64-bit), Elf (32/64-bit), Binary, Windows PE (32/64-bit) and DOS MZ.
precompiled JWasm binaries are available for DOS, Windows and Linux. For OS/2 and FreeBSD, makefiles are supplied.
Instructions up to AVX are supported.
JWasm is written in C. The source is portable and has successfully been tested with Open Watcom, MS VC, GCC and more.
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.

JWasm started as a fork of Open Watcom's Wasm in March 2008. Today, the part of Wasm source lines still contained in JWasm is approximately 20%.

Additional information:

JWasm's Readme.
Manual, describes the differences between JWasm and Masm. For a good description of Masm's syntax and features, see Art of Assembly Language Programming.
Table of pros and cons of JWasm compared to other Masm-style assemblers.
Symbolic Debugging of assembly code with
- MS WinDbg, 32- and 64-bit Windows.
- Pelle Orinius' POIDE, 32- and 64-bit Windows.
- Open Watcom's WDW, 16- and 32-bit Windows, 16-bit DOS.
- Open Watcom's WD, 16- and 32-bit DOS.
- MS CodeView, 16-bit DOS.
How to use JWasm with MS Visual Studio 2005/2008/2010/2012

JWasm uses SourceForge for user feedback:

Pipeline for next releases:

		Binary Windows	Binary Linux	Binary DOS	Source
JWasm v2.10 stable	04/20/2013	JWasm210bw.zip	JWasm210bl.zip	JWasm210bd.zip	JWasm210s.zip	change log

WRC - OW resource compiler v1.9	05/2010	Win32	wrc.zip (binary)
WDW - modified OW Windows debugger v1.9, may debug Win32, Win16 and DOS apps locally	01/2013	Win32	wdw.zip (binary)
WD - OW DOS debugger v1.9	05/2010	DOS	wd.zip (binary)
PE - PE, COFF, AR dumper; may dump CV4-CV8 debug info in COFF files	03/15/2013	Win32, DOS	pe.zip (binary)
Agner Fog's ObjConv v2.09 - Object format converter + Disassembler	08/2009	DOS	objconv.zip (binary)
MS OMF Linker v5.60	12/1994	Win32, DOS	~~lnk563.exe (MS FTP host)~~	lnk563.exe (MS HTTP)
MS NMake v1.5	1994	Win32	~~nmake15.exe (MS FTP host)~~