At one time it was 800px
; however, more and more sites are beginning to use 1000px
. This is driven by the fact that, more often than not, visitors' low end resolutions are at 1024x768
. The additional 24 pixels leaves room for the browser chrome, padding and margins you may want to manipulate, etc.
Just to give some perspective, even the new wave of netbooks with their tiny screen have resolutions of 1024x768
.
But, above all else, know your audience as @warren suggests and capitalize on that.