Horizontal global navigation often isn’t practical because the horizontal space can hold only a handful of top-level options. For example, what if your information hierarchy has 12 or 15 top-level items? You could display global navigation vertically down the left side of every page, but on lower-level pages that uses space that could otherwise be used for a page-specific navigation bar.
Walmart has implemented vertical global navigation in a way that also works for lower-level pages. On the home page, the global navigation bar is persistently expanded:
On all pages other than the home page, the global navigation bar is present, but displays only on rollover:
Amazon.com uses a similar approach – at least for now (pending the launch of their new site design).