HX DOS extender
I'm preserving Japheth's MS-DOS extender. It features 16bit, and 32bit capabilities with the ability to run 16bit OS/2 program, and 32bit Win32 code on Vanilla MS-DOS.
Virtual Drivers for the NT Virtual DOS Machine implementing clipboard access and joystick emulation.
Japheth's assembler, 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.
MicroThread is a minimal Borland/Turbo C multi-threading library for DOS programs. It consists of a pre-emptive scheduling kernel which dispatch threads on DOS INT 8, and has support for semaphores, critical sections and inter-thread messages.
Development environment for OS/2 and DOS
FarSo is a Dos multitasker GUI, actualy in portuguese and in PASCAL. Currenctly its inactive. The project was rewrited to C++ and used as the GUI of http://sourceforge.net/projects/dccnitghtmare/