Link awareness is defined as the ability to discover, view, search and update global hyperlink information about any resource with a URL on the World Wide Web. This global link information is a shared information resource.
Implementing link awareness is difficult. In practice, an implementation only approximates link awareness. There are at least two qualitative axes on which we can classify these implementations.
Breadth of Coverage: The number and type of interlinked documents covered.
Liveness: The ability (or lack thereof) to update the link information on the fly.
Most WikiWiki implementations support link awareness within the scope of the documents that they host. Their breadth of coverage (insofar as link information is concerned) is narrow. A wiki, however, is very live (and indeed, liveness, is its raison d'etre).
Google supports link awareness through its web interface. (You type in a query of the form link:<URL>.) Google's breadth is impressive (it covers a lot of interlinked documents), but it is a far cry from live (it can take months before Google indexes a newly created document).