The old D86 debugger[1][2] comes close to being a REPL for assembly language, helped me a lot with learning it when I found it on a shareware collection CD as a kid.

Load registers, call DOS or BIOS with 'int', etc. all interactively and with a nice full screen display of registers, flags and memory. Of course entering single instructions to run immediately only gets you so far, but you can also enter short programs into memory and single step through them.

It's too bad nothing like this seems to exist for modern systems! With the CPU virtualization features now available, you could even experiment with ring 0 code without fear of crashing your machine.

[1] http://eji.com/a86

[2] to run it under DOSBox, you may need to add the '+b4' command line switch in order to bypass some machine detection code