UPDATE - This is now redundant as Firefox and Chrome have built in functionality to tell you window width etc.
I spend ages when doing RWD re-sizing the browser and finding parts that need tweaking with media queries, but I never had a solid way of working out what the current width was (using Firebug Inspector took ages!)..
So I created this (obviously this requires jQuery):
if(e.keyCode == 45)
Press Insert whenever you need the innerWidth :)