The days of 800x600 screens are all but over, but most of us still try to accommodate 1024px wide screens. Hence the popularity of "960" width sites. This bookmarklet will resize the current browser window to that width and height. You know, so us web designers with giant monitors can see what it's like to be slummin' with a 1024 screen. Also, to see "the fold", if such a concept even matters anymore.
1024x768 < Drag to bookmarks bar
There is some kind of way to do it, since I still regularly see resized popups, so feel free to chime in the comments if you know more.