Google Browser Size helps you see what your site's users see
Google Labs recently released Browser Size, a browser window size visualization tool. Utilizing browser window size data from Google's home page (and thus monitoring a huge sample size of the world, much less your site's visitors), to overlay statistics, in terms of percentages of users, on top of your site (or any site you choose to enter). This data reflects visible area, not actual window size. That is, it subtracts all the toolbar fluff and lets you see what your users actually see.
This tool is very useful (if extremely ugly) for ensuring that important calls-to-action on your site are plainly visible to the widest audience possible. You can adjust the translucency of the overlay and it even allows you to interact normally with your page with the overlay on top. Nicely done, Browser Size team!