- herkforth is a native, colored Forth for the PPC and was written by Jason Woofenden.
- Fronds is a rewrite of herkforth for a simple virtual machine, so it can easily run on just about any hardware and under any OS.
- gforth is a free (GNU GPL) and quite popular ANS Forth. Many of the Forth code examples have been tested with it.
- colorForth is an operating system for x86 PC's. It was written by Chuck Moore, the creator of Forth!
- FICL (Forth Inspired Command Language) is a programming language interpreter designed to be embedded into other systems as a command, macro, and development prototyping language. It is very solid and easy-to-use. http://ficl.sourceforge.net/
- PygmyForth is a free Forth implementation for DOS, based on cmForth and F-PC. It is a great learning environment.
- BashForth is a free Forth implementation, written as Bash script. More about BashForth can be found in http://wikipedia.org/wiki/Bashforth.
- RetroForth a public domain Forth for DOS, Windows, GNU/Linux, FreeBSD, and is used as a native operating system for the x86. More information is at http://www.retroforth.org.
- SEDForth is a free forth compiler written as a sed script. It generates Linux/x86 assembly language using subroutine threading and some easy optimizations. It started as an offshoot of RetroForth.
- 4IM is a standalone Forth system for x86. It also runs under DOS and uses tail recursion like colorForth. Well featured. More at http://4im.atspace.com/.
- isforth is an x86 only Forth compiler for GNU/Linux.
- 4tH is a Forth compiler for GNU/Linux, Windows, BeOS, DOS, AIX, FreeBSD, Solaris and many other Unixes. It is licensed under the LGPL and is written in ANSI-C. It generates bytecode, C-embeddable bytecode, C-programs, but also works fine as a scripting language. The entire compiler can be embedded in a C-program. Written by Hans Bezemer.
- Glypher is a colored derivative of RetroForth using the Allegro graphics library. Its creator has moved on to do a Forth implementation for the Nintendo GameCube, but a binary is still available at http://retroforth.org/glypher/.
- JForth is a (free) complete Forth programming environment for the Classic Amiga systems. It has an online manual, FAQ, links, and programs. See the site for more information. http://jforth.org/
- Please add more...
Here's a map of a bunch of Forth implementations, and how they relate to each other: http://www.complang.tuwien.ac.at/forth/family-tree/ It doesn't have much from the past five years, but is otherwise very cool.