common screen sizes

Here is a quick rundown of some of the most common screen sizes:

Smartphones: 240x320 to 640x960, most commonly 480x800px
Tablets: 480x800 to 600x800, 600x1024, 768x1024, or 720x1280 -- the iPad is 1024x768px
Laptops: 800x600 to 1600x1200, even 1920x1200 -- the most common being 1024x768px
Desktops: 1024x768 all the way up to 1920x1200. The most common size is 1280x1024px.
Given so many different screen resolutions and differing aspect ratios, what on Earth is a designer to do?