Technically fine, but wouldn't this be better as an improvement to the existing Calculator? (and isn't there any better way than calling out to gcalctool everytime I press a key?)

Depends. As you say, this requires calling out to gcalctool, maybe some people don't care for that and would rather just have the javascript version. That said, I guess I could offer it to the existing Calculator tool. The main advantage of using gcalctool instead of javascript is you get access to trig functions, exponentiation, logarithms, etc. Maybe some people won't need to use the calculator for that. I believe I also read somewhere that the precision of javascript math isn't very good, probably not a big deal for some people though. As to the calling gcalctool, I have a primitive regex in place that requires there to at least be digits present. If it is a big deal, that could be modified to something more complex. All that is required is changing the _validExpression method. However, I'm not really sure what you would consider sufficient conditions to call the program. If you have a better idea, I'd be happy to implement it.

This is fine with me.