Google labs has just released an interesting tool call Browser Size. You can use it at: http://browsersize.googlelabs.com/

Basically this online tool allows you to get a glimpse on how webpage content will be viewed on different screen resolutions. I know that there are many plugins for various browsers that exist that can do this, but to my knownlegde, none show you all the different resolution views at the same time.

"Google Browser Size is a visualization of browser window sizes for people who visit Google. For example, the "90%" contour means that 90% of people visiting Google have their browser window open to at least this size or larger.

"This is useful for ensuring that important parts of a page's user interface are visible by a wide audience. On the example page that you see when you first visit this site, there is a "donate now" button which falls within the 80% contour, meaning that 20% of users cannot see this button when they first visit the page. 20% is a significant number; knowing this fact would encourage the designer to move the button much higher in the page so it can be seen without scrolling."

http://browsersize.googlelabs.com/static/about-browser-size.html