Gforth is a fast and portable implementation of the ANS Forth language. It works nicely with the Emacs editor, offers some nice features such as input completion and history and a powerful locals facility, and it even has (the beginnings of) a manual. Gforth employs traditional implementation techniques: its inner innerpreter is indirect or direct threaded. Gforth runs under Unix, Win95, OS/2, and DOS and should not be hard to port to other systems supported by GCC.
4tH is a Forth compiler with a little difference. Instead of the standard Forth engine it features a conventional compiler. 4tH is a very small compiler that can create bytecode, C-embeddable bytecode, standalone executables, but also works fine as a scripting language. It supports about 95% of the ANS Forth CORE wordset and features conditional compilation, pipes, files, assertions, forward declarations, enumerations, structures, suspended execution, recursion, include files, etc. It comes with an RPN calculator, line editor, preprocessor, compiler, decompiler, C-source generator, a virtual machine, and a multitasking environment.
amforth is an extendible command interpreter for the Atmel AVR ATmega microcontroller family. It has a turnkey feature for embedded use as well. It does not depend on a host application. The command language is an almost compatible ANS94 forth with extensions. It needs less than 8KB code memory for the base system. It is written in assembly language and forth itself.
Michael4 is a Forth-like language implemented for the 64-bit Alpha Linux platform. It was tested on gentoo Alpha. It differs slightly from normal Forths in that you can call words that are not yet defined and they are automatically stubbed if the source code file doesn't exist for them.
Minimac is a minimalist, general purpose text macro processor. Its simplicity should make it particularly well suited as a front end preprocessor for little language compilers. It is meant to be simpler to use than m4. It uses an explicit argument stack, and user functions are defined by concatenation (similar to the Forth language). Macro expansion is delayed to the last possible moment. The software is currently in alpha release.