Achernar - 11:14 pm on Sep 8, 2007 (gmt 0)

I've been doing this for a couple of months now, and it appears that more and more people a browsing with sidebars and with the browser's window not maximized.

I log:
screen.width+'x'+screen.height
screen.availWidth+'x'+screen.availHeight (1)
window.outerWidth+'x'+window.outerHeight (2)
window.innerWidth+'x'+window.innerHeight (3)

and since IE doesn't have (2)(3), I'm logging this instead:
document.documentElement.scrollWidth+'x'+document.documentElement.scrollHeight (4)

with (3) or (4) I can see when a user opens a second tab (or a sidebar) and keeps browsing the site.
with (2) I see if a visitor has decided to maximize the window, or has changed it's shape.