So I haven't reallly looked into this until you mentioned it, but there is XDebug and there is an eclipse client called PDT that supports XDebug. This lets you

So I haven't reallly looked into this until you mentioned it, but there is XDebug and there is an eclipse client called PDT that supports XDebug. This lets you do gdb style things like put watches on variables, halt execution, change the value of variables at runtime etc. This might be in line with what you are talking about. I don't know how many languages XDebug supports but if I had to come up with a solution from scratch, I believe you would need to hack the respective interpreters for your languages to get the functionality you are suggesting. Once you get the ability to externally inform the interpreter of how you want it to behave (other than just executing the program it normally does) you can get watches, breakpoints and changes variables on the fly.