I have just started poking with forth, after seeing some network card firmware source written in it.

One of the most interesting things, to me, was the built-in disassembler: if you ask 'how is this word defined', and it's not defined in the current environment, it'll go disassemble the binary and show you that.

Pretty insane, if you think about having to write software to interface with old / undocumented / closed source libraries.