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.
Factor is a dynamically-typed stack-based programming language. Factor runs on various OSes and CPU architectures, and provides an optimizing compiler, interactive development environment, powerful collections, higher order programming, continuations, and a growing library of contributed code.
FreeForth is a small and fast interactive compiler composed of an extensible set of macros generating inline compact i386 native code, including floating-point instructions, and an easy interface to Linux and Windows dynamic libraries. It uses two stacks to pass subroutines arguments and results separately from return addresses, like other Forth dialects, but unlike them, it is a simpler pure compiler (without an interpreter) offering interactivity through "anonymous" subroutines which are executed by their closing ";" macro. FreeForth is fully documented by 100K of interactive online help. Since its first release in 2006, it has been used every day for cross-development of realtime industrial applications embedded in microcontrollers, and for PC-controlled manufacturing test benches. The FreeForth distribution includes an interactive incremental assembler for the MSP430 microcontroller family.
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.
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.