I was actually curious as to what people use for their everyday web browsing... not what size they develop websites for, which is an entirely different question.
I am also assuming that they may or may not use the maximum resolution that their system is capable of, but just whatever it is that they find most comfortable for surfing the web.
I think that for most websites, it's probably wise to go for the 800x600 as that will cover everyone.... but then you pretty much have to use the downward scrolling style to fit in enough information on a single page.
I've decided to design for 1024x768 (the next larger standard size) because I don't want to use the screen scroll and need more screen area for content... plus, anyone looking for advanced audio products for computer midi composition and sound design most likely has a hi-rez monitor and is using it... which also makes an 800x600 site look very small - unless one uses clever designs that disguise the fact that only about 700 pixels of horizontal space is being used for actual content.
Anyway... just a few comments on screen resolution...
For my customer I started with 800 x 600 but with all the extras inside the browser window (e.g. yahoo tool bar, button bar inside firefox) the space for the page is simply to limited. I decided to switch to 1024 x 768.