For some time now, I’m a big fan of the Window Size Bookmarklet by Joss Crowcroft. This bookmarklet allows you to display the X/Y size of the viewport in the left corner of your browser. It can be very useful for debugging some CSS3 media queries.
When I came across on responsive.is, I said to myself: “Victor, you must write your own bookmarklet which mix both features.”
I mean a bookmarklet which allows to display the size of the viewport and some buttons to change the viewport to smartphone/tablet size automatically.