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, backtraces, a
decompiler and a powerful locals facility, and it even has a
manual. Gforth combines traditional implementation techniques with
newer techniques for portability and performance performance: its
inner interpreter is direct threaded with several optimizations, but
you can also use a traditional-style indirect threaded interpreter.