One of the most useful, and often overlooked features of a website is the search box.
Most visitors to your site will land on your home page but more often than not are seeking information stored on different pages. If you intend on having a site with a lot of content then the internal search box is a valuable tool to allow your visitors to quickly locate the page they need rather than manually scanning numerous menu options.
A tagging system is another way of directing visitors to pages that cover similar topics to the one they are currently on. Each article would display a small ammount of tags (keywords relevant to the article) that link the visitor to other articles sharing the same tag.
Both systems aid site navigation and make larger sites much easier to explore.