There are still plenty of sites that are built to work only with IE for Windows, but now that the alternatives are so good and the advantages increasingly obvious, this is changing.
It's been a long time coming, but part of the reason Firefox is so exciting is that web designers are finally starting to come around to the value of web standards (namely XHTML, CSS, and ECMAScript). ESPN.com was one of the first of the really huge sites to commit to these standards, and the trend is accelerating. See Jeffrey Zeldman's excellent book Designing with Web Standards to get a handle on these developments. And if you haven't already seen it, here's a cool article about how to get
/. into full standards compliance.