Comment by pgeorgi

OpenFirmware, such as the various implementations on http://www.openbios.org/ always comes with a Forth prompt.

I was part of the team that wrote the original Open Source implementation (under GPL terms) named OpenBIOS. The project now also hosts all kinds of other implementations that were later published under BSD terms by their owners (and had 10+ years of market experience under their belt at that time).

When FORTH started out, one of its differentiators was its live environment: you could test one step (part of an algorithm or similar) at the prompt, then define a "word" (= function) that implements it using the statements you tried, repeat until you finished your program using the words you defined earlier.