Site maps can make users’ life easier in many situations, for example when they look for specific information they don’t find on the homepage.
"The site map is an alternative way of navigation…"
A good website design should have a site map and a link to it in a visible location. The site map is an alternative way of navigation, often use as a last resource. One possible approach is to organize it by categories representing group of products, but there are many solutions that would work fine.
Apple.com has a well design site map, the link to it is at the bottom of the homepage:
EBay has an extended site map and the link to it it’s very visible, at the top right corner:
Mozilla.com does not pass this usability test as they don’t provide a site map (or maybe I can’t find it, can you?).