For those who want to play around I have extended Matthew's code with
some enhancements to allow
1) dynamically add lines with p
2) dump memory and stack -->nstruction ?d
3) show the stack --> ?s
4) Halt execution until <Enter> is pressed  --> ?h
5) Enabled comment mode with ?=  thus = this is ignored = to make
programs (even LOL) more readable

I found it quite helpful for debugging my Befunge programs
here it is, but the credit goes to Matthew of course.

http://pastie.org/324874

Enjoy
Robert