I've been using between 900 and 1000 recently. Most 15" monitors are about 1280x1024 or higher. So anything less than 1024px will fit.
Yeah I usually create fixed width in the range of 960 - 990... I find that some 15" screens at 1024 have some issues with the scroll bar on certain browsers so I try to stay under 1000.. ==> As stated above, there is no standard pixel width.... Cheers!~
According to my experience 990px has no issues with any browser. If you will go higher than this then you will face horizontal scroll bar issue.
It depends what monitor resolutions you want to take into account, personally I would probably go with around 960 as has been suggested because 800x600 is pretty outdated but you want to take into account 1024x768 In a few years we probably will be in the same position with 1024x768px resolutions..but for now you want to take this into account as horizontal scrolling is pretty annoying for the user. You might want to consider a pretty background for very large res monitors as the layout can look pretty skinny on them! Of course this is for fixed width websites you might want to consider fluid widths using %'s so that it re-sizes according to the viewport width.
960 is a good standard today. The added advantage of 960 is that it divides by 2, 3, 4, 5, 6, 8, 10, 12, 15, 16, 20, 24 ....... This allows you a lot of easy layout choices.
According to www.w3schools.com Browser Display Statistics there were 20% of PCs with 1024 x 768 px in use (Jan. 2010). So it makes sense to take them into account.
I always go with a liquid or actually a semi liquid design. This way the page always fits the browser and rather than coding to the lowest common denominator you can make your page "size" to whatever browser / monitor it is being displayed on.
I almost always use 960 width, centered on the page. There is no standard. And if you think this is fun, try formatting for mobile devices.