Many websites will include a tiny link to their sitemap — an index of their site’s content — down in the bottom right corner. Sitemaps are a great way for users to find their way trough content-rich sites, a great accessibility feature, and the most effective way to ensure that search engines is crawling the entire website. This really should be featured more prominently in web designs.
Sitemaps should absolutely be included in the top level, or main, navigation of most sites. Especially content-rich sites which is often hard to navigate, even with cleaver categorization, and good navigational menus.
Navigating to the sitemap should be made easier but getting there is only half the battle. The tricky part is navigating the sitemap it self!
A sitemap should provide links to all top-level and sub-level content categories. As well as all product categories, documentation, and support pages. It might also be a good idea to include a “top ten most popular pages segment.”
The best practices for sitemap layout is freely open for discussion, but some general things should be easy to agree on. Like that the sitemap should not be table based (everyone seams to be doing them table based!), it should not be a too high page (to avoid too much scrolling), and that all top categories should be emphasized.
And a nifty, little tip here at the end: if there are more than forty links on a sitemap it will become pretty cluttered. Consider separating the sitemap into two main categories on separate pages or provide a basic and an advanced sitemap.