Most websites use forward indices in some form or another for site navigation. However, some systems like a wiki can use the more automatic ReverseIndex. Nonetheless, a ForwardIndex has the advantage of being edited. That is, people have selected the targets and organized them in some manner that is suitable for TheAudience. In this way, a ForwardIndex is reader oriented.
A ForwardIndex gives the categorisation decision to the author of the category page, rather than the author of the topic page. This ensures consistent categorisation, but can lead to more errors as the category author may lack the specialised knowledge of the topic author. For example, EnglishWikipedia's short-lived "list of famous heterosexuals" erroneously included Casanova, who was in fact bisexual. Forward indices subtly encourage "big bang" categorisations, where a new category is created and hundreds of pages are quickly added to it.
A BidirectionalIndex is both a forward and a reverse index.
Beware of overusing forward indexes, or overloading them with too many links: readers may feel InformationOverload from a HubAndSpoke construction. Also, without BackLinks from the pages referred to, a reader will lose her sense of place in the document, so a ForwardIndex by itself is often undesirable (rather a BidirectionalIndex is preferrable). Due to wikis conflation of text with links via the LinkPattern, a page's BackLinks do not locate it within the ForwardIndex necessarily, as the BackLinks include pages that mention it as well as categorize it.
AllPages is the quintessential ForwardIndex for many sites, but beware of using AllPages as your front page since it is very long and completely flat, so you lose the main advantage of using a hypermedium: organization of information.