There are other tools out there for testing to see how your site will look in different browsers, but this is Google’s effort. It is a very simple interface which allows you to test what percentage of visitors will see what proportion of your site.
In other words, it is a tool that shows (unsurprisingly) browser sizes in relation to your site:

Yes, the lines look a little scrappy, but Google claims that they actually visually represent the proportions of visitors in terms of who will see what as determined by Google’s own visitor stats.
“So what?” I hear you cry. Surely our web guys would be using a tool like this already, and shouldn’t designers know about which browser sizes to optimize for anyway?
Well, here’s the clever bit, in Google’s own words:
“Think the numbers are different for your site? Browser Size bases its data on a sample of visitors to Google.com, but you can check out your screen resolutions report in your web analytics tool to see the most common visitor screen resolutions for your own site. For most sites the list of dimensions is quite long, and the ones we in the industry tend to use – 1024×768, 1280×800, 1280×1024 – typically only represent about 50% of the visitors at best.”
There is also an indication that this may well be integrated as a feature into one or more of the Google products, so it may be a good idea to get used to seeing it if you use Analytics, or Optimizer.
Using it in with your own stats may well lead to you being able to optimize an extra few % out of your site…
You can check out the tool here: http://browsersize.googlelabs.com/